So far, we've only looked at whole numbers. We've had no way to represent parts of numbers or numbers in between the points on a numberline. Rational numbers give us the ability to represent points in between the whole numbers. To understand the points between whole numbers we need to look at the concewpt of infinity.
Infinity is a number that is so large or so small that it can't be represented, but we know it exists. Something can be infinitly large meaning it's so big there isn't a number that can be written to represent it. There are an infinite number of whole numbers because for any whole number you can add 1 to it and come up with another whole number.
Everyone has heard the joke routine of a person counting down to an ultimatum. I'm going to count to 5 and then I'm going to ... Then the person counts; one, two ,three, four, four and one half, four and three quarters... This joke works because there are an infinite number of parts bewteen 4 and 5. Have you ever wondered why the football never crosses the goalline when a team is third and goal and the defense gets charged with a penalty that results in half the distance to the goal? That's because you can always cut the distacne to the goal in half and get closer to the goal, but you never actually cross the goal.
Cutting something in half can;t be represented by whole numbers because half isn't a whole anymore. We use something called rational numbers to represent halves. Rational numbers are also called fractions because they represent fractions of a whole. Rational numbers are ratios of a number to a whole. One half is a ratio of 1 to 2. It is also a fraction of 1 part of 2 parts of a whole.
A rational number is made up of two parts. The number is represented as a number over another number. The number on top, called the numerator, is the part of the whole that we're talking about. The number on the bottom, called the denominator, is the number of parts that the whole is divided into. In the number 1/2, we are talking about 1 part of 2. Two halves make a whole.
The number 1/2 is between 0 and 1 on the numberline.
<——0——½——1——>
If we divide the distance between 0 and 1 into fourths we get this numberline
<——0—¼—½—¾—1——>
Notice how we count up the fourths from 1 to 3. At 1, we would be at 4 fourths. When the numerator equals the denominator, we are at 1 or a whole. If we continued to count fourths passed 1 we would get 5/4, 6/4, 7/4, 8/4.
Notice that 8 is a multiple of 4. 8/4 is the same as 2. Also notice that the gcd of 6 and 4 is 2. If we divide 6 and 4 by 2 we get 3 and 2. 6/4 is the same as 3/2. If the numerator and denominator of a rational number have a gcd, we have to use this gcd to divide the rational number to its reduced form. We never represent rational numbers with a gcd between the numerator and denominator. Rational numbers must always be represented in their reduced form.
Anytime the denominator is 1, the reduced number is a whole number. 4/1 would be 4 wholes or just 4. Anytime you see a rational number where the denomintaor is 1, simply drop the denominator.
The numberline divided into fourths would look like this
<——0—1/4—1/2—3/4—1—5/4—3/2—7/4—2—9/4—5/2—11/4—3——>
Multiplying ratinoal numbers is easy. You simply multiply the numerators and multiply the denominators. Then reduce the rational number by removing the gcd of the numeratoar and denominator, if it exists.
3/2×4/3 = (3×4)/(2×3) = 12/6
The gcd of 12 and 6 is 6, so divide the numerator by 6 and divide the denominator by 6
(12÷6)/(6÷6) = 2/1 = 2
Adding rational numbers requires us to add numbers with common denominators. To get a common denominators, we have to have the lcm of the denominators. To refresh the concept of the lcm look at the blog Division Revisited.Then we change each rational number so it looks different, but isn't really changed. We can do this by multiplying each number by 1. Remember that in multiplication, 1 is the identity meaning that when we multiply by 1 we don't change the value of the number. From above, if the numerator and denominator are the same, the rational number is actually 1. So we are going to multiply each of the rational numbers by a rational number where the numerator and the denominator are the same. This changes the way the rational numbers look but not their actual value. This is confusing, so let's work one out. Let's add 1/2 to 1/3.
First, we need the lcm of 2 and 3. The lcm of 2 and 3 is 6. So we need to convert 1/2 and 1/3 to sixths. If we multiply 2 by 3 we get 6, so we multiply 1/2 by 3/3 to get 3/6. If we multiply 3 by 2 we get 6, so we multiply 1/3 by 2/2 to get 2/6. Now that we have a common denominator, we simply add the numerators.
1/2+1/3 = 3/6+2/6 = (3+2)/6 = 5/6
So to add two rational numbers, we first have to find the lcm of the denominators, multiply both numbers by 1 to get a common denominator, then add the numerators and reduce if needed.
Subtraction works like addition. Get a common denominator then subtract the second numerator from the first.
5/6-1/3 = 5/6 - 2/6 = (5-2)/6 = 3/6 = 1/2
Division works similar to multiplication. But we flip the divisor then we multiply the numerators and the denominators and reduce if necessary.
Rational numbers give us the ability to represent the inverse for multiplication. Remember from addition that the inverse is the number that we add to a number to get the identity which is 0 in addition. In multiplication, the identity is 1, so we want the number that when we multiply we get 1. In multiplication, the inverse is called the reciprocal and we get it by flipping the rational number. The reciprocal of 1/2 is 2/1, and if we multiply 1/2 by 2/1 we get (1×2)/(2×1) = 2/2 = 1. When we multiplied by the reciprocal, we got the identity. So the reciprocal is the inverse.
Rational numbers actually represent a division problem and we'll investigate this further in the next blog.
Saturday, June 9, 2007
Saturday, June 2, 2007
Division Revisited
Just as subtraction is the opposite of addition, division is the opposite of multiplication. We learned that multiplication is adding a number a multiple of times. Division, likewise, is subtracting multiple times. We start with the dividend and we subtract the divisor until we get to zero or we can't subtract any more.
6-2=4-2=2-2=0
6÷2=3
9-2=7-2=5-2=3-2=1
Subtracting by 2 again would go past zero.
9÷2=4 with a remainder of 1 or 9=2×4+1
What happens when we divide by negative numbers? We divide just as we would with positive numbers, but we have to remember our sign rules for multiplication. Remember that an odd number of negative signs in multiplication gives a negative number. So if we multiply two negative numbers, we get a positive product. If we multiply two positive numbers we get a positive product. So if we divide a positive number, we must have gotten it by multiplying two numbers with the same sign. That means, in division, the divisor and the quotient must have the same sign. If the dividend starts negative, then the divisor and quotient must have different signs.
8÷-2=-4
The dividend is positive, the divisor gives the sign of the quotient.
-8÷-2=4
-8÷2=-4
The dividend is negative, the quotient has the opposite sign of the divisor.
9÷-2=-4 with a remainder of 1 because 9=-2×-4 + 1
-9÷-2=4 with a remainder of -1 because -9=-2×4 - 1
Let's look at division of large numbers.
12345÷567
Just start subtracting by 567 and keep count.
12345-567=11778 (1st division)
11778-567=11211 (2nd division)
11211-567=10644 (3rd division)
10644-567=10077 (4th division)
10077-567=9510 (5th division)
9510-567=8943 (6th division)
8943-567=8376 (7th division)
8376-567=7809 (8th division)
7809-567=7242 (9th division)
7242-567=6675 (10th division)
6675-567=6108 (11th division)
6108-567=5541 (12th division)
5541-567=4974 (13th division)
4974-567=4407 (14th division)
4407-567=3840 (15th division)
3840-567=3273 (16th division)
3273-567=2706 (17th division)
2706-567=2139 (18th division)
2139-567=1572 (19th division)
1572-567=1005 (20th division)
1005-567=438 (21st division)
So 12345÷567=21 with a remainder of 438
Doing division by multiple subtractions can be tedious and presents lots of opportunites for errors. We start by looking only at the number in the highest positions. So we want to divide 10000 by 500. Since there are two zeros on the right of 5, we can eliminate two zeros from the right in both numbers. So now we are looking at 100÷5. From our multiplication table, we don't have a multiple of 5 that equals 100. But we do have a multiple of 10 that equals 100 and a multiple of 5 that equals 10. 5×2×10=100. Using the associativity of the integers, we have 5×(2×10)=5×20=100. Putting the zeros back in, we have that 10000÷500=20.
Start by multiplying 567 by 20
12345-11340=1005
1005 > 567 so multiply by 21 instead.
12345-11907=438
438 < 567
So 12345÷567=21 with a remainder of 438
This is essentially how the Indians developed long division. Their notation is different, though. We're going to use the ) notation of long division with the divisor to the left and the dividend to the right. We are going to write our quotient above the dividend, carfeully observing positioning. The result at the bottom will be the remainder. We start like this:
Now we count off positions in the dividend until we get a number that is larger than the divisor. 1234 is larger than 567. How many times does 5 go into 12? 2 times. We place 2 above the 4 in 1234. We will multiply 567 by 2 and place that product underneath 1234. Then we subtract that from 1234 and place the difference underneath. It looks like this.
Now we bring down the 5 in the next column of 12345 to make a number greater than 567. How many times does 5 go into 10? 2 times. Place the 2 above the 5. Multiply 567 by 2 and place the product underneath 1005.
1134 is greater than 1005, so we reduce the multiplier by 1. Place 1 above the 5 and multiply 567 by 1. Place 567 underneath 1005 and subtract.
438 < 567 so we're done.
12345=567×21+438
Division is neither associative nor commutative. The grouping and the order of the numbers matters.
(18÷6)÷3 = 3÷3 = 1
18÷(6÷3) = 18÷2 = 9
Finally, there is a special number that we look at regarding division. It will become very important in the next blog. It is the greatest common divisor. The greatest common divisor, or gcd, is the largest number that evenly divides two numbers. We look at the prime number divisors of a two numbers and collect the divisors they have in common.
You get the prime factors of a number by dividing it by prime numbers until you get to 1. If the number is even then you can divide by 2, for example, 246 is even and 246÷2=123. If a number ends in 5 then you can divide dy 5, for example, 85 ends in 5 and 85÷5=17. If the sum of the digits of a number is a multiple of 3 then the number can be divided by 3, for example, in 123 the digits add up to 6 which is a multiple of 3 and 123÷3=41. Here's the url in Wikipedia that lists the divisibility rules. http://en.wikipedia.org/wiki/Divisibility_rule
The prime factors of 8 are 2,2 and 2. The prime factors of 12 are 2,2 and 3. Both 8 and 12 have a pair of 2's in common. So the greatest common divisor of 8 and 12 is 2×2=4. Another way to find the gcd is to multiply the two numbers together and divide by the lcm. You can similarly find the lcm by multiplying the two numbers together and divide by the gcd. The product of 8 and 12 is 96. The lcm of 8 and 12 is 24. 96 divided by 24 is 4.
6-2=4-2=2-2=0
6÷2=3
9-2=7-2=5-2=3-2=1
Subtracting by 2 again would go past zero.
9÷2=4 with a remainder of 1 or 9=2×4+1
What happens when we divide by negative numbers? We divide just as we would with positive numbers, but we have to remember our sign rules for multiplication. Remember that an odd number of negative signs in multiplication gives a negative number. So if we multiply two negative numbers, we get a positive product. If we multiply two positive numbers we get a positive product. So if we divide a positive number, we must have gotten it by multiplying two numbers with the same sign. That means, in division, the divisor and the quotient must have the same sign. If the dividend starts negative, then the divisor and quotient must have different signs.
8÷-2=-4
The dividend is positive, the divisor gives the sign of the quotient.
-8÷-2=4
-8÷2=-4
The dividend is negative, the quotient has the opposite sign of the divisor.
9÷-2=-4 with a remainder of 1 because 9=-2×-4 + 1
-9÷-2=4 with a remainder of -1 because -9=-2×4 - 1
Let's look at division of large numbers.
12345÷567
Just start subtracting by 567 and keep count.
12345-567=11778 (1st division)
11778-567=11211 (2nd division)
11211-567=10644 (3rd division)
10644-567=10077 (4th division)
10077-567=9510 (5th division)
9510-567=8943 (6th division)
8943-567=8376 (7th division)
8376-567=7809 (8th division)
7809-567=7242 (9th division)
7242-567=6675 (10th division)
6675-567=6108 (11th division)
6108-567=5541 (12th division)
5541-567=4974 (13th division)
4974-567=4407 (14th division)
4407-567=3840 (15th division)
3840-567=3273 (16th division)
3273-567=2706 (17th division)
2706-567=2139 (18th division)
2139-567=1572 (19th division)
1572-567=1005 (20th division)
1005-567=438 (21st division)
So 12345÷567=21 with a remainder of 438
Doing division by multiple subtractions can be tedious and presents lots of opportunites for errors. We start by looking only at the number in the highest positions. So we want to divide 10000 by 500. Since there are two zeros on the right of 5, we can eliminate two zeros from the right in both numbers. So now we are looking at 100÷5. From our multiplication table, we don't have a multiple of 5 that equals 100. But we do have a multiple of 10 that equals 100 and a multiple of 5 that equals 10. 5×2×10=100. Using the associativity of the integers, we have 5×(2×10)=5×20=100. Putting the zeros back in, we have that 10000÷500=20.
Start by multiplying 567 by 20
| 5 | 6 | 7 | ||
| × | 2 | 0 | ||
| 1 | 1 | 3 | 4 | 0 |
12345-11340=1005
1005 > 567 so multiply by 21 instead.
| 5 | 6 | 7 | ||
| × | 2 | 1 | ||
| 1 | 1 | 9 | 0 | 7 |
12345-11907=438
438 < 567
So 12345÷567=21 with a remainder of 438
This is essentially how the Indians developed long division. Their notation is different, though. We're going to use the ) notation of long division with the divisor to the left and the dividend to the right. We are going to write our quotient above the dividend, carfeully observing positioning. The result at the bottom will be the remainder. We start like this:
| — | — | — | — | — | ||||
| 5 | 6 | 7 | ) | 1 | 2 | 3 | 4 | 5 |
Now we count off positions in the dividend until we get a number that is larger than the divisor. 1234 is larger than 567. How many times does 5 go into 12? 2 times. We place 2 above the 4 in 1234. We will multiply 567 by 2 and place that product underneath 1234. Then we subtract that from 1234 and place the difference underneath. It looks like this.
| 2 | ||||||||
| — | — | — | — | — | ||||
| 5 | 6 | 7 | ) | 1 | 2 | 3 | 4 | 5 |
| - | 1 | 1 | 3 | 4 | ||||
| — | — | — | — | |||||
| 1 | 0 | 0 |
Now we bring down the 5 in the next column of 12345 to make a number greater than 567. How many times does 5 go into 10? 2 times. Place the 2 above the 5. Multiply 567 by 2 and place the product underneath 1005.
| 2 | 2 | |||||||
| — | — | — | — | — | ||||
| 5 | 6 | 7 | ) | 1 | 2 | 3 | 4 | 5 |
| - | 1 | 1 | 3 | 4 | ||||
| — | — | — | — | |||||
| 1 | 0 | 0 | 5 | |||||
| - | 1 | 1 | 3 | 4 | ||||
| — | — | — | — |
1134 is greater than 1005, so we reduce the multiplier by 1. Place 1 above the 5 and multiply 567 by 1. Place 567 underneath 1005 and subtract.
| 2 | 1 | |||||||
| — | — | — | — | — | ||||
| 5 | 6 | 7 | ) | 1 | 2 | 3 | 4 | 5 |
| - | 1 | 1 | 3 | 4 | ||||
| — | — | — | — | |||||
| 1 | 0 | 0 | 5 | |||||
| - | 5 | 6 | 7 | |||||
| — | — | — | — | |||||
| 4 | 3 | 8 |
438 < 567 so we're done.
12345=567×21+438
Division is neither associative nor commutative. The grouping and the order of the numbers matters.
(18÷6)÷3 = 3÷3 = 1
18÷(6÷3) = 18÷2 = 9
Finally, there is a special number that we look at regarding division. It will become very important in the next blog. It is the greatest common divisor. The greatest common divisor, or gcd, is the largest number that evenly divides two numbers. We look at the prime number divisors of a two numbers and collect the divisors they have in common.
You get the prime factors of a number by dividing it by prime numbers until you get to 1. If the number is even then you can divide by 2, for example, 246 is even and 246÷2=123. If a number ends in 5 then you can divide dy 5, for example, 85 ends in 5 and 85÷5=17. If the sum of the digits of a number is a multiple of 3 then the number can be divided by 3, for example, in 123 the digits add up to 6 which is a multiple of 3 and 123÷3=41. Here's the url in Wikipedia that lists the divisibility rules. http://en.wikipedia.org/wiki/Divisibility_rule
The prime factors of 8 are 2,2 and 2. The prime factors of 12 are 2,2 and 3. Both 8 and 12 have a pair of 2's in common. So the greatest common divisor of 8 and 12 is 2×2=4. Another way to find the gcd is to multiply the two numbers together and divide by the lcm. You can similarly find the lcm by multiplying the two numbers together and divide by the gcd. The product of 8 and 12 is 96. The lcm of 8 and 12 is 24. 96 divided by 24 is 4.
The Alphabet of Math: The integers and subtraction
We can do some simple forms of math using only the natural numbers and zero. The natural numbers and zero are called the whole numbers. Right now we haven't developed the notion of pieces of numbers, but since we call the natural numbers and zero the whole numbers, you can guess that we are going to eventually talk about numbers that aren't whole. We will, but one step at a time.
All of our numbers, so far, have fit on a number line that starts at zero and moves to the right. But what happens if you want to start at zero and move to the left? The number line can be extended to the left from zero using the concept of negative numbers. We simply number from zero, counting up just like we do on the right, but we preface each number with a negative sign to indicate it is to the left of zero.
-6···-5···-4···-3···-2···-1···0···1···2···3···4···5···6
This extended set of numbers (the natural numbers, zero and the negative numbers) are called the integers. The integers with a negative sign are called negative numbers and those without a negative sign are called positive. We can do addition and multiplication on positive numbers just as before. But, since negative numbers are the opposite of positive numbers, when we encounter them in addition or multiplication, we change our direction and move to the left. If an addend is positive, move to the right. If an addend is negative, move to the left.
Let's look at some examples of addition using negative numbers.
6+ -2. Start at 6 and move to the left by 2.
-6···-5···-4···-3···-2···-1···0···1···2···3···4···5···6
6+ -2 =4
-6+ 2. Start at -6 and move to the right by 2.
-6···-5···-4···-3···-2···-1···0···1···2···3···4···5···6
-6+ 2 =-4
6+ -8. Start at 6 and move to the left by 8.
-6···-5···-4···-3···-2···-1···0···1···2···3···4···5···6
6+ -8=-2
8+ -6. Start at 8 and move to the left by 6.
-6···-5···-4···-3···-2···-1···0···1···2···3···4···5···6···7···8
8+ -6=2
Notice the similarity between 8+ -6 and 6+ -8. They both equal 2 but the signs are different. Notice how in 6+ -8 the answer has the sign of the number with the higher magnitude. The same thing happened with 6+ -2 and -6+2. The answer is still 4, but with the sign is the same as the number with the higher magnitude.
This process of adding by moving to the left instead of the right is called subtraction. Subtraction is the opposite of addition. In addition, you start at a number and move to the right. In subtraction, you start at a number and move to the left. Negative signs tend to have more power than a positive sign, so when you see them side by side, the negative sign takes over. Because of this, 6+ -8 is written as 6-8 and 6+ -2 is written as 6-2.
What happens if we start with a negative number and subtract?
-4-2 = -6
-6···-5···-4···-3···-2···-1···0···1···2···3···4···5···6
When the signs are the same, you add the numbers and keep the sign. When the signs are different, you subtract the numbers and keep the sign of the larger number.
The integers inherit the commutativity and associativity of the natural numbers. Zero is still the identity of addition. With the integers, addition gets another special number called an inverse. The inverse is a number that you add by to get the identity. (Multiplication also has an inverse, but we need another type of number that will come later.) So the inverse of 4 is -4 because 4-4=0. The inverse of -4 is 4 because -4+4=0.
How about multiplication of negative numbers? We need to add a step to our multiplication process to account for negative multiplicands. If the multiplicand (the number on the left) is positive, we face to the right. If the multiplicand is negative, we face to the left. If the multiplier is positive we jump forward. If the multiplier is negative we jump backward.
3×-3
Start facing to the right and jump backwards.
························································>
-9···-8···-7···-6···-5···-4···-3···-2···-1···0···1···2···3···4···5···6···7···8···9
3×-3=-9
-3×3
Start facing to the left and jump forwards.
························································<
-9···-8···-7···-6···-5···-4···-3···-2···-1···0···1···2···3···4···5···6···7···8···9
-3×3=-9
-3×-3
Start facing to the left and jump backwards.
························································<
-9···-8···-7···-6···-5···-4···-3···-2···-1···0···1···2···3···4···5···6···7···8···9
-3×-3=9
Notice when the signs are different, the answer is negative and when the signs are the same the answer is positive. If you are multiplying a string of numbers and the number of negative signs is odd the answer is always negative.
So far we have been looking at what happens when we add or subtract two numbers. We can also compare two numbers. We look at the relative position of the two numbers on the number line and determine which one is further to the left or right. If a number is further to the right than the other, it is said to be greater than the other. If a number is further to the left than the other, it is said to be less than the other. If two numbers are at the same place on the number line, then they are said to be equal to each other. There is a law in mathematics called the trichotomy law. It says that a number can compare to another number in only one of three ways (tri is Greek for three). It is either greater than, less than or equal to the other number. Greater than is denoted by < and less than is denoted by >. The arrow always points to the smaller number.
3 < 6 and -3 > -6.
It gets confusing with negative numbers so you have to mentally place the numbers on the number line and look at which number is further to the right. That number is the greater number. On a number line, -3 is further to the right than -6, so -3 is greater than -6.
Comparisons are not usually done on individual numbers. You are more likely to see them in reference to two equations. You simplify each equation separately, then compare the resulting numbers.
3+6 < 3×4
9 < 12
4×4 = 12+4
16 = 16
3×1-2 > 3+4×-2
3-2 > 3-8
1 > -5
Now that we have the operation of subtraction, let's revisit division in the next blog.
All of our numbers, so far, have fit on a number line that starts at zero and moves to the right. But what happens if you want to start at zero and move to the left? The number line can be extended to the left from zero using the concept of negative numbers. We simply number from zero, counting up just like we do on the right, but we preface each number with a negative sign to indicate it is to the left of zero.
-6···-5···-4···-3···-2···-1···0···1···2···3···4···5···6
This extended set of numbers (the natural numbers, zero and the negative numbers) are called the integers. The integers with a negative sign are called negative numbers and those without a negative sign are called positive. We can do addition and multiplication on positive numbers just as before. But, since negative numbers are the opposite of positive numbers, when we encounter them in addition or multiplication, we change our direction and move to the left. If an addend is positive, move to the right. If an addend is negative, move to the left.
Let's look at some examples of addition using negative numbers.
6+ -2. Start at 6 and move to the left by 2.
-6···-5···-4···-3···-2···-1···0···1···2···3···4···5···6
6+ -2 =4
-6+ 2. Start at -6 and move to the right by 2.
-6···-5···-4···-3···-2···-1···0···1···2···3···4···5···6
-6+ 2 =-4
6+ -8. Start at 6 and move to the left by 8.
-6···-5···-4···-3···-2···-1···0···1···2···3···4···5···6
6+ -8=-2
8+ -6. Start at 8 and move to the left by 6.
-6···-5···-4···-3···-2···-1···0···1···2···3···4···5···6···7···8
8+ -6=2
Notice the similarity between 8+ -6 and 6+ -8. They both equal 2 but the signs are different. Notice how in 6+ -8 the answer has the sign of the number with the higher magnitude. The same thing happened with 6+ -2 and -6+2. The answer is still 4, but with the sign is the same as the number with the higher magnitude.
This process of adding by moving to the left instead of the right is called subtraction. Subtraction is the opposite of addition. In addition, you start at a number and move to the right. In subtraction, you start at a number and move to the left. Negative signs tend to have more power than a positive sign, so when you see them side by side, the negative sign takes over. Because of this, 6+ -8 is written as 6-8 and 6+ -2 is written as 6-2.
What happens if we start with a negative number and subtract?
-4-2 = -6
-6···-5···-4···-3···-2···-1···0···1···2···3···4···5···6
When the signs are the same, you add the numbers and keep the sign. When the signs are different, you subtract the numbers and keep the sign of the larger number.
The integers inherit the commutativity and associativity of the natural numbers. Zero is still the identity of addition. With the integers, addition gets another special number called an inverse. The inverse is a number that you add by to get the identity. (Multiplication also has an inverse, but we need another type of number that will come later.) So the inverse of 4 is -4 because 4-4=0. The inverse of -4 is 4 because -4+4=0.
How about multiplication of negative numbers? We need to add a step to our multiplication process to account for negative multiplicands. If the multiplicand (the number on the left) is positive, we face to the right. If the multiplicand is negative, we face to the left. If the multiplier is positive we jump forward. If the multiplier is negative we jump backward.
3×-3
Start facing to the right and jump backwards.
························································>
-9···-8···-7···-6···-5···-4···-3···-2···-1···0···1···2···3···4···5···6···7···8···9
3×-3=-9
-3×3
Start facing to the left and jump forwards.
························································<
-9···-8···-7···-6···-5···-4···-3···-2···-1···0···1···2···3···4···5···6···7···8···9
-3×3=-9
-3×-3
Start facing to the left and jump backwards.
························································<
-9···-8···-7···-6···-5···-4···-3···-2···-1···0···1···2···3···4···5···6···7···8···9
-3×-3=9
Notice when the signs are different, the answer is negative and when the signs are the same the answer is positive. If you are multiplying a string of numbers and the number of negative signs is odd the answer is always negative.
So far we have been looking at what happens when we add or subtract two numbers. We can also compare two numbers. We look at the relative position of the two numbers on the number line and determine which one is further to the left or right. If a number is further to the right than the other, it is said to be greater than the other. If a number is further to the left than the other, it is said to be less than the other. If two numbers are at the same place on the number line, then they are said to be equal to each other. There is a law in mathematics called the trichotomy law. It says that a number can compare to another number in only one of three ways (tri is Greek for three). It is either greater than, less than or equal to the other number. Greater than is denoted by < and less than is denoted by >. The arrow always points to the smaller number.
3 < 6 and -3 > -6.
It gets confusing with negative numbers so you have to mentally place the numbers on the number line and look at which number is further to the right. That number is the greater number. On a number line, -3 is further to the right than -6, so -3 is greater than -6.
Comparisons are not usually done on individual numbers. You are more likely to see them in reference to two equations. You simplify each equation separately, then compare the resulting numbers.
3+6 < 3×4
9 < 12
4×4 = 12+4
16 = 16
3×1-2 > 3+4×-2
3-2 > 3-8
1 > -5
Now that we have the operation of subtraction, let's revisit division in the next blog.
Deconstruction: Division
Division is the opposite of multiplication. In multiplication, we start at zero and jump to the right a given number of times to the answer. In division we start on a number and jump to the left to zero counting the number of jumps. In division, the number of jumps to zero is the answer.
We start at 6 and start jumping by 2's to the left towards zero. We count each jump.
0-----1-----2-----3-----4-----5-----6
It took three jumps to get to zero, so 6÷2=3.
The numbers involved in division are called the dividend (the number we start with), the divisor (the number we subtract by or the size of the jumps we make) and the answer is called the qoutient. Let's divide 6 by 2. The equation is written 6÷2. 6 is the dividend and 2 is the divisor.
You can learn the result of division for some common small numbers by using the multiplication table in reverse. Find the dividend in the middle of the chart with the proper divisor on the left, then follow the column to the top to find the quotient.
What happens if the jumps don't exactly land on zero. The left over is called the remainder. Let's try 9÷2 and see what happens.
0-----1-----2-----3-----4-----5-----6-----7-----8-----9
We made 4 jumps to the left and landed on one. That means 9÷2 is 4 with a remainder of 1. What we are actually saying is that 4×2+1=9. (Remember from our order of operations that multiplication is carried out before addition.) This illustrates the division algorithm. Any whole number is the product of a whole number and a quotient plus a remainder.
To divide larger numbers, we need another method of deconstructing numbers. And for that method we need to extend our alphabet to include a new type of number. We need the operation of subtraction and we need the concept of negative numbers to do subtraction. We will introduce negative numbers in the next blog and use the operation of subtraction to extend division to large numbers.
We start at 6 and start jumping by 2's to the left towards zero. We count each jump.
0-----1-----2-----3-----4-----5-----6
It took three jumps to get to zero, so 6÷2=3.
The numbers involved in division are called the dividend (the number we start with), the divisor (the number we subtract by or the size of the jumps we make) and the answer is called the qoutient. Let's divide 6 by 2. The equation is written 6÷2. 6 is the dividend and 2 is the divisor.
You can learn the result of division for some common small numbers by using the multiplication table in reverse. Find the dividend in the middle of the chart with the proper divisor on the left, then follow the column to the top to find the quotient.
What happens if the jumps don't exactly land on zero. The left over is called the remainder. Let's try 9÷2 and see what happens.
0-----1-----2-----3-----4-----5-----6-----7-----8-----9
We made 4 jumps to the left and landed on one. That means 9÷2 is 4 with a remainder of 1. What we are actually saying is that 4×2+1=9. (Remember from our order of operations that multiplication is carried out before addition.) This illustrates the division algorithm. Any whole number is the product of a whole number and a quotient plus a remainder.
To divide larger numbers, we need another method of deconstructing numbers. And for that method we need to extend our alphabet to include a new type of number. We need the operation of subtraction and we need the concept of negative numbers to do subtraction. We will introduce negative numbers in the next blog and use the operation of subtraction to extend division to large numbers.
Sunday, April 15, 2007
Building Simple Words: Multiplication
Multiplication is the process of multiple additions. Multiplication is shorthand for multiple addition operations. Like addition, multiplication is a binary operation, meaning there are two terms. The first number is called the multiplicand and the second is called the multiplier. The result is called the product. The numbers that make up a product are also called the factors.
Multiplication is the process of adding multiple times. For example,
2 × 3 would be the same as 2+2+2=6.
Multiplication can be performed on the number line by starting at 0 and skipping to the right to the answer. Multiplying 2 by 3 would require starting at 0 then skipping to the right by 2's three times. The first skip lands on 2, the second skip lands on 4 and the third skip lands on 6.
0--1--2--3--4--5--6--7--8--9--10->
Just as we can develop a table for addition, we can develop a table for multiplication.
To find the product of two numbers, we start with the multiplicand at the top and read down the column to the row starting with the multiplier. Where the two meet is the product.
It is useful to memorize the multiplication table, just as it is useful to memorize the addition table.
Multiplying large numbers requires multiplication by each column and then addition of those multiples.
We start by multiplying 1357 by 6. Any multiple that is greater than 9 carries the tens digit to the next column to the left as an add after the multiplication of that column. The colored number will be the carry.
7 × 6=42
We keep the ones value and carry the tens value to the next column.
5 × 6=30+4=34
We keep the ones value and carry the tens value to the next column.
3 × 6=18+3=21
We keep the ones value and carry the tens value to the next column.
1 × 6=6+2=8
So our first row of multiplication is 8142.
Now we multiply 1357 by 40. Since we are multiplying by a multiple of 10, we place a 0 in the ones column to hold the position open, then simply multiply by 4.
7 × 4=28
We keep the ones value and carry the tens to the next column.
5 × 4=20+2=22
We keep the ones value and carry the tens value to the next column.
3 × 4=12+2=14
We keep the ones value and carry the tens value to the next column.
1 × 4=4+1=5
So our second row of multiplication is 54280. Notice the 0 holding the ones position open.
Now we multiply 1357 by 200. Since we are multiplying by a multiple of 100, we place a 0 in both the ones and the tens column to hold the position open and multiply by 2.
7 × 2=14
We keep the ones value and carry the tens to the next column.
5 × 2=10+1=11
We keep the ones value and carry the tens to the next column.
3 × 2=6+1=7
We don't have a carry this time.
1 × 2=2
So our third row of multiplication is 271400. Notice the zeros holding the tens and ones positions open.
Now we add up the three rows of multiples.
So 1357 × 246=333822. That's 1357 added to itself 246 times. We had to make three multiplications and one addition to get the answer. That's a lot faster than doing 246 additions.
Just like with addition, multiplication is commutative; 3 × 2=2 × 3. And, just like with addition, multiplication is also associative;(3 × 2)× 4=3 ×(2 × 4). And, just like with addition, there is also an identity element in multiplication; it is 1. Multiplying a number by 1 doesn't change its identity. This will be an important tool later on. Also, notice that multiplying any number by 0 gives 0. There is an important rule to remember in mathematics that if the product of two numbers is 0, one of them must have been 0 to begin with.
Addition and multiplication can be mixed in a sentence. When we see addition and multiplication together, there is a particular order that we do the operations in. In English, "cats eat birds" means something completely different if we change the order to "birds eat cats." In math, the order of the operations can change the meaning of a mathematical sentence. Look at the sentence 2+3 × 4 for example.What if we add 2 and 3 then multiply by 4. We get 5 × 4=20. But what if we multiply 3 and 4 first, then add 2. We get 2+12=14. In math, multiplication is always done before addition. So 2+3 × 4 is 14. To avoid confusion, we use parenthesis around operations. Operations inside parenthesis are done first. So it would be clear that 2+(3 × 4) is 14 and (2+3)× 4=20.
When mixing multiplication and addition, there is a special property called distribution, when we have multiplication on the outside of a parenthesis containing an addition. In something like
2×(3+4), the multiplication distributes to each term of the addition inside the parenthesis.
2×(3+4) is the same as 2 × 3+2 × 4. It is clear that 2 × 7=14 and 6+8=14. We will use this distributive property a lot in the future.
There is a special multiple that will become important later on. It is called the least common multiple or lcm. It is the smallest number that is a common factor of two numbers. You start with the prime factorization of two numbers, then you count the each factor at least once.
Some rules for factoring numbers are as follows. If the number is even, 2 is a factor. If the number ends in 5, then 5 is a factor, If the sum if the digits of a number add up to a multiple of 3, then 3 is a factor. There are other rules that can be found at Wikipedia http://en.wikipedia.org/wiki/Divisibility_rule
Let's find the least common multiple of 48 and 52. If we just look at the product, 48×52=2496. Now let's factor. The factors of 48 are 2,2,2,2 and 3. The factors of 52 are 2,2, and 13. The lcm of 48 and 52 would be the product of four 2's, a 3 and a 13. 2×2×2×2×3×13=624. The reason the lcm is less than the straight product of 48 and 52 is that we don't have to multiply by the extra 2's in 52; we already counted them in the factors of 48. The smallest number that is a factor of both 48 and 52 is 48×13=624 and 52×12=624.
Multiplication is the process of adding multiple times. For example,
2 × 3 would be the same as 2+2+2=6.
Multiplication can be performed on the number line by starting at 0 and skipping to the right to the answer. Multiplying 2 by 3 would require starting at 0 then skipping to the right by 2's three times. The first skip lands on 2, the second skip lands on 4 and the third skip lands on 6.
0--1--2--3--4--5--6--7--8--9--10->
Just as we can develop a table for addition, we can develop a table for multiplication.
| × | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 1 | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
| 2 | 0 | 2 | 4 | 6 | 8 | 10 | 12 | 14 | 16 | 18 | 20 |
| 3 | 0 | 3 | 6 | 9 | 12 | 15 | 18 | 21 | 24 | 27 | 30 |
| 4 | 0 | 4 | 8 | 12 | 16 | 20 | 24 | 28 | 32 | 36 | 40 |
| 5 | 0 | 5 | 10 | 15 | 20 | 25 | 30 | 35 | 40 | 45 | 50 |
| 6 | 0 | 6 | 12 | 18 | 24 | 30 | 36 | 42 | 48 | 54 | 60 |
| 7 | 0 | 7 | 14 | 21 | 28 | 35 | 42 | 49 | 56 | 63 | 70 |
| 8 | 0 | 8 | 16 | 24 | 32 | 40 | 48 | 56 | 64 | 72 | 80 |
| 9 | 0 | 9 | 18 | 27 | 36 | 45 | 54 | 63 | 72 | 81 | 90 |
| 10 | 0 | 10 | 20 | 30 | 40 | 50 | 60 | 70 | 80 | 90 | 100 |
To find the product of two numbers, we start with the multiplicand at the top and read down the column to the row starting with the multiplier. Where the two meet is the product.
It is useful to memorize the multiplication table, just as it is useful to memorize the addition table.
Multiplying large numbers requires multiplication by each column and then addition of those multiples.
| 1 | 3 | 5 | 7 | |
| × | 2 | 4 | 6 |
We start by multiplying 1357 by 6. Any multiple that is greater than 9 carries the tens digit to the next column to the left as an add after the multiplication of that column. The colored number will be the carry.
7 × 6=42
We keep the ones value and carry the tens value to the next column.
5 × 6=30+4=34
We keep the ones value and carry the tens value to the next column.
3 × 6=18+3=21
We keep the ones value and carry the tens value to the next column.
1 × 6=6+2=8
So our first row of multiplication is 8142.
| 1 | 3 | 5 | 7 | |
| × | 2 | 4 | 6 | |
| 8 | 1 | 4 | 2 |
Now we multiply 1357 by 40. Since we are multiplying by a multiple of 10, we place a 0 in the ones column to hold the position open, then simply multiply by 4.
7 × 4=28
We keep the ones value and carry the tens to the next column.
5 × 4=20+2=22
We keep the ones value and carry the tens value to the next column.
3 × 4=12+2=14
We keep the ones value and carry the tens value to the next column.
1 × 4=4+1=5
So our second row of multiplication is 54280. Notice the 0 holding the ones position open.
| 1 | 3 | 5 | 7 | |
| × | 2 | 4 | 6 | |
| 8 | 1 | 4 | 2 | |
| 5 | 4 | 2 | 8 | 0 |
Now we multiply 1357 by 200. Since we are multiplying by a multiple of 100, we place a 0 in both the ones and the tens column to hold the position open and multiply by 2.
7 × 2=14
We keep the ones value and carry the tens to the next column.
5 × 2=10+1=11
We keep the ones value and carry the tens to the next column.
3 × 2=6+1=7
We don't have a carry this time.
1 × 2=2
So our third row of multiplication is 271400. Notice the zeros holding the tens and ones positions open.
| 1 | 3 | 5 | 7 | ||
| × | 2 | 4 | 6 | ||
| 8 | 1 | 4 | 2 | ||
| 5 | 4 | 2 | 8 | 0 | |
| 2 | 7 | 1 | 4 | 0 | 0 |
Now we add up the three rows of multiples.
| 8 | 1 | 4 | 2 | |||
| 5 | 4 | 2 | 8 | 0 | ||
| + | 2 | 7 | 1 | 4 | 0 | 0 |
| 3 | 3 | 3 | 8 | 2 | 2 |
So 1357 × 246=333822. That's 1357 added to itself 246 times. We had to make three multiplications and one addition to get the answer. That's a lot faster than doing 246 additions.
Just like with addition, multiplication is commutative; 3 × 2=2 × 3. And, just like with addition, multiplication is also associative;(3 × 2)× 4=3 ×(2 × 4). And, just like with addition, there is also an identity element in multiplication; it is 1. Multiplying a number by 1 doesn't change its identity. This will be an important tool later on. Also, notice that multiplying any number by 0 gives 0. There is an important rule to remember in mathematics that if the product of two numbers is 0, one of them must have been 0 to begin with.
Addition and multiplication can be mixed in a sentence. When we see addition and multiplication together, there is a particular order that we do the operations in. In English, "cats eat birds" means something completely different if we change the order to "birds eat cats." In math, the order of the operations can change the meaning of a mathematical sentence. Look at the sentence 2+3 × 4 for example.What if we add 2 and 3 then multiply by 4. We get 5 × 4=20. But what if we multiply 3 and 4 first, then add 2. We get 2+12=14. In math, multiplication is always done before addition. So 2+3 × 4 is 14. To avoid confusion, we use parenthesis around operations. Operations inside parenthesis are done first. So it would be clear that 2+(3 × 4) is 14 and (2+3)× 4=20.
When mixing multiplication and addition, there is a special property called distribution, when we have multiplication on the outside of a parenthesis containing an addition. In something like
2×(3+4), the multiplication distributes to each term of the addition inside the parenthesis.
2×(3+4) is the same as 2 × 3+2 × 4. It is clear that 2 × 7=14 and 6+8=14. We will use this distributive property a lot in the future.
There is a special multiple that will become important later on. It is called the least common multiple or lcm. It is the smallest number that is a common factor of two numbers. You start with the prime factorization of two numbers, then you count the each factor at least once.
Some rules for factoring numbers are as follows. If the number is even, 2 is a factor. If the number ends in 5, then 5 is a factor, If the sum if the digits of a number add up to a multiple of 3, then 3 is a factor. There are other rules that can be found at Wikipedia http://en.wikipedia.org/wiki/Divisibility_rule
Let's find the least common multiple of 48 and 52. If we just look at the product, 48×52=2496. Now let's factor. The factors of 48 are 2,2,2,2 and 3. The factors of 52 are 2,2, and 13. The lcm of 48 and 52 would be the product of four 2's, a 3 and a 13. 2×2×2×2×3×13=624. The reason the lcm is less than the straight product of 48 and 52 is that we don't have to multiply by the extra 2's in 52; we already counted them in the factors of 48. The smallest number that is a factor of both 48 and 52 is 48×13=624 and 52×12=624.
Saturday, April 14, 2007
Building Simple Words: Addition
Because there is an order to the natural numbers, we can lay them out on a number line. It looks like this:
0---1---2---3---4---5---6---7---8---9---10--->
Starting with a number on the number line, we can get to another number on the number line by stepping to the right on the line. This process is called addition. We take two numbers and add them together to form another number. For this reason, addition is called a binary (meaning two) operation. The two numbers are called the addends and the result of the operation is called the sum.
We work our way from left to right along the number line. For example, let's add 3 + 2.
Start on 3 on the number line and step to the right 2 times.
0---1---2---3---4---5---6---7---8---9---10--->
We stop on 5. So 3 + 2 = 5.
We can build a table of the results of adding the numbers 0 to 9 and memorize the results
To add two numbers, we find the first number on the top row and follow the column down to the row with the second number on the left. Where the column and row meet is the sum of the two numbers. For example, 7 + 6 = 13. Start with 7 on the top and follow the column down to the row that starts with 6. Where the column and row meet is 13.
Adding larger numbers requires lining up the positions in columns. All of the ones line up under each other; all of the tens line up under each other; all of the hundreds line up under each other and so on. Then you add down the columns starting with the ones column and move to the left. If the sum of the column is more than 9, you keep the ones value from the sum and add 1 to the column on the left. The number that is "carried" to the column on the left is called the carry.
Let's add 1357 + 246.
Line the numbers up in columns
Starting with the ones column adding down the column: 7+6=13. Keep the 3 in the ones column and add 1 to the tens column. The carry is in color.
Now add down the tens column: 1+5+4=10. Keep the 0 in the tens column and add 1 to the hundreds.
Now add down the hundreds column: 1+3+2=6.
Since there is no number in the thousands column in the second number, we fill in the position with a 0. Now add down the thousands column: 1+0=1
Adding more than two numbers at a time, you still line up the numbers in columns and add down the columns starting with the ones column. You can only add two numbers at a time, but you can keep a running total as you add your way down the column. When you get to the bottom, keep the ones value and carry the rest of the number to the columns on the left. If your carry is larger than 9, the carry overflows to the columns on the left, carrying to more than one column if you have to.
Addition is the simplest way to form larger numbers. As you can see from the addition table, there are several ways to get to other numbers: 3+2=5 and 4+1=5. You might also notice that changing the order of the numbers doesn't make a difference: 3+2=2+3. This is called commutativity and is a basic property of the natural numbers.
When adding more than tow numbers, we use parenthesis around numbers to indicate the order we want to add. Numbers inside of parenthesis get added first, then that sum gets added to the numbers on the outside of the parenthesis. 3+(4+2) means add 4+2 then add 3 to that sum.
3+(4+2)=
3+ 6 = 9.
This way we can group addition together.
Another basic property of the natural numbers is associativity. This means the grouping doesn't matter. If we add more than two numbers at a time, it doesn't matter how we group the numbers together:
3+(4+2)=(3+4)+2
3+ 6 = 7 +2
9 = 9
These basic properties will follow us all the way through the alphabet. As we add more types numbers to our alphabet, those numbers will also be commutative and associative. Remember, commutative means order and associative means group. An easy way to remember it is that when a governor commutes a sentence he changes the court's order and an association is a group of people. This ability of numbers to commute and associate is a very powerful tool when we start looking at much more complicated forms of math.
In addition, 0 is a very special number. As you can see, anytime we add 0 to a number, the number doesn't change. 0 is called the identity element, because addition by 0 doesn't change a number's identity. This will become an important tool to use later on. The ability to change a number's look without changing its value is very useful.
0---1---2---3---4---5---6---7---8---9---10--->
Starting with a number on the number line, we can get to another number on the number line by stepping to the right on the line. This process is called addition. We take two numbers and add them together to form another number. For this reason, addition is called a binary (meaning two) operation. The two numbers are called the addends and the result of the operation is called the sum.
We work our way from left to right along the number line. For example, let's add 3 + 2.
Start on 3 on the number line and step to the right 2 times.
0---1---2---3---4---5---6---7---8---9---10--->
We stop on 5. So 3 + 2 = 5.
We can build a table of the results of adding the numbers 0 to 9 and memorize the results
| + | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
| 0 | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
| 1 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
| 2 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 |
| 3 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
| 4 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 |
| 5 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 |
| 6 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 |
| 7 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 |
| 8 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 |
| 9 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 |
To add two numbers, we find the first number on the top row and follow the column down to the row with the second number on the left. Where the column and row meet is the sum of the two numbers. For example, 7 + 6 = 13. Start with 7 on the top and follow the column down to the row that starts with 6. Where the column and row meet is 13.
Adding larger numbers requires lining up the positions in columns. All of the ones line up under each other; all of the tens line up under each other; all of the hundreds line up under each other and so on. Then you add down the columns starting with the ones column and move to the left. If the sum of the column is more than 9, you keep the ones value from the sum and add 1 to the column on the left. The number that is "carried" to the column on the left is called the carry.
Let's add 1357 + 246.
Line the numbers up in columns
| 1 | 3 | 5 | 7 |
| + | 2 | 4 | 6 |
Starting with the ones column adding down the column: 7+6=13. Keep the 3 in the ones column and add 1 to the tens column. The carry is in color.
| 1 | |||
| 1 | 3 | 5 | 7 |
| + | 2 | 4 | 6 |
| 3 |
Now add down the tens column: 1+5+4=10. Keep the 0 in the tens column and add 1 to the hundreds.
| 1 | |||
| 1 | 3 | 5 | 7 |
| + | 2 | 4 | 6 |
| 0 | 3 |
Now add down the hundreds column: 1+3+2=6.
| 1 | 3 | 5 | 7 |
| + | 2 | 4 | 6 |
| 6 | 0 | 3 |
Since there is no number in the thousands column in the second number, we fill in the position with a 0. Now add down the thousands column: 1+0=1
| 1 | 3 | 5 | 7 |
| + | 2 | 4 | 6 |
| 1 | 6 | 0 | 3 |
Adding more than two numbers at a time, you still line up the numbers in columns and add down the columns starting with the ones column. You can only add two numbers at a time, but you can keep a running total as you add your way down the column. When you get to the bottom, keep the ones value and carry the rest of the number to the columns on the left. If your carry is larger than 9, the carry overflows to the columns on the left, carrying to more than one column if you have to.
Addition is the simplest way to form larger numbers. As you can see from the addition table, there are several ways to get to other numbers: 3+2=5 and 4+1=5. You might also notice that changing the order of the numbers doesn't make a difference: 3+2=2+3. This is called commutativity and is a basic property of the natural numbers.
When adding more than tow numbers, we use parenthesis around numbers to indicate the order we want to add. Numbers inside of parenthesis get added first, then that sum gets added to the numbers on the outside of the parenthesis. 3+(4+2) means add 4+2 then add 3 to that sum.
3+(4+2)=
3+ 6 = 9.
This way we can group addition together.
Another basic property of the natural numbers is associativity. This means the grouping doesn't matter. If we add more than two numbers at a time, it doesn't matter how we group the numbers together:
3+(4+2)=(3+4)+2
3+ 6 = 7 +2
9 = 9
These basic properties will follow us all the way through the alphabet. As we add more types numbers to our alphabet, those numbers will also be commutative and associative. Remember, commutative means order and associative means group. An easy way to remember it is that when a governor commutes a sentence he changes the court's order and an association is a group of people. This ability of numbers to commute and associate is a very powerful tool when we start looking at much more complicated forms of math.
In addition, 0 is a very special number. As you can see, anytime we add 0 to a number, the number doesn't change. 0 is called the identity element, because addition by 0 doesn't change a number's identity. This will become an important tool to use later on. The ability to change a number's look without changing its value is very useful.
Friday, April 13, 2007
The Alphabet of Math: The counting numbers
Just like any language, math has an alphabet. We will build the alphabet in steps. The basic alphabet is the natural or counting numbers. The natural numbers occur in nature. They are also called the counting numbers because they are numbers used to count things. Just like the English alphabet, there is an order. In order, the natural numbers are 1,2,3,4,5,6,7,8,9.
According to The History of Numbers, when math started out, there were only three numbers; one, two and many. There are so called "primitive" cultures alive in the world today where those are still the only numbers used. One, two and many isn't very precise. So many has to be quantified.
According to The History of Numbers, people naturally began counting on fingers. That was good for five or ten things, but what if you had more things to count? Then you could count two more things by counting arms, two more by counting legs, two more by counting eyes or ears. In some primitive cultures, it is possible to count a large number of items by ticking off the fingers and various other body parts. To know if you have lost any sheep during the day, just remember what part of the body you counted up to in the morning and make sure you get that far at the end of the day.
The most commonly developed scheme for keeping track of numbers is using marks on a piece of wood; one mark for each item. Grouping marks, say into fives or tens, makes it easy to see at a glance how many items you have. With practice, you learn to recognize multiples of five or ten. The Sumerians were the first to use symbols for numbers, but they were really only sophisticated groups of tick marks. Counting on body parts is good for a relatively small number of items. The Sumerians were able to represent truly large numbers in the millions to keep track of grain harvests and distribution of resources and even do some astronomy which calls for REALLY big numbers.
The numbers that we use today come from India through Arabia. With the addition of the concept of zero, which we also get from the Indians, we can represent any number. We start counting at 0, denoting an absence of items and count from 1 to 9. To represent larger numbers, we borrow a concept from the Sumerians called positional notation. As we go above 9, we add numbers to the left. Each position to the left represents a multiple of 10. Each position to the left gets a new name. From right to left they go ones, tens, hundreds, thousands, tens of thousands, hundreds of thousands, millions and so on. So 25 represents 2 tens and 5. The number 347 is 3 hundreds, 4 tens and 7.
With the alphabet from 1 to 9 and positional notation, it is possible to represent any number we can count, no matter how large. With this alphabet, we are ready to learn how to combine numbers together to form larger numbers. We'll discuss that in my next blog on basic arithmetic.
According to The History of Numbers, when math started out, there were only three numbers; one, two and many. There are so called "primitive" cultures alive in the world today where those are still the only numbers used. One, two and many isn't very precise. So many has to be quantified.
According to The History of Numbers, people naturally began counting on fingers. That was good for five or ten things, but what if you had more things to count? Then you could count two more things by counting arms, two more by counting legs, two more by counting eyes or ears. In some primitive cultures, it is possible to count a large number of items by ticking off the fingers and various other body parts. To know if you have lost any sheep during the day, just remember what part of the body you counted up to in the morning and make sure you get that far at the end of the day.
The most commonly developed scheme for keeping track of numbers is using marks on a piece of wood; one mark for each item. Grouping marks, say into fives or tens, makes it easy to see at a glance how many items you have. With practice, you learn to recognize multiples of five or ten. The Sumerians were the first to use symbols for numbers, but they were really only sophisticated groups of tick marks. Counting on body parts is good for a relatively small number of items. The Sumerians were able to represent truly large numbers in the millions to keep track of grain harvests and distribution of resources and even do some astronomy which calls for REALLY big numbers.
The numbers that we use today come from India through Arabia. With the addition of the concept of zero, which we also get from the Indians, we can represent any number. We start counting at 0, denoting an absence of items and count from 1 to 9. To represent larger numbers, we borrow a concept from the Sumerians called positional notation. As we go above 9, we add numbers to the left. Each position to the left represents a multiple of 10. Each position to the left gets a new name. From right to left they go ones, tens, hundreds, thousands, tens of thousands, hundreds of thousands, millions and so on. So 25 represents 2 tens and 5. The number 347 is 3 hundreds, 4 tens and 7.
With the alphabet from 1 to 9 and positional notation, it is possible to represent any number we can count, no matter how large. With this alphabet, we are ready to learn how to combine numbers together to form larger numbers. We'll discuss that in my next blog on basic arithmetic.
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