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Finding All the Factors of a Number

Date: 09/10/2001 at 22:14:40

From: Shaya

Subject: I need help with factors

What are the factors of 1-100? Like 1 = 1x1, and so on.

Date: 09/11/2001 at 10:48:54

From: Doctor Ian

Subject: Re: I need help with factors

Hi Shaya,

Do you know what a prime number is? A number is prime if it cannot be

divided evenly by anything except itself and 1. For example, 5 is a

prime number, because the only factors of 5 are

1 * 5 = 5

However, 12 is _not_ a prime number, because

1 * 12 = 12

2 * 6 = 12

3 * 4 = 12

Why should you care about prime numbers? For one thing, they make iteasy to find the factors of a number.

Let's consider a number like 84. It's even, so it's divisible by 2:

84 = 2 * 42

Now, what about 42? That's also even, so it's divisible by 2:

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84 = 2 * 2 * 21

What about 21? That's not divisible by 2, but it _is_ divisible by 3:

84 = 2 * 2 * 3 * 7

And 7 is prime. So what we've done here is find the 'prime factors' of

the number 84.

How does this help us find _all_ the factors of 84? Well, if a number

is a factor of 84, and it's not one of these prime factors, then it

must be a product of two or more prime factors. So we can find all the

factors of 84 by looking at all the ways we can group these prime

factors:

2 * 2 = 4 pairs

2 * 3 = 6

2 * 7 = 14

3 * 7 = 21

2 * 2 * 3 = 12 triplets

2 * 2 * 7 = 28

2 * 3 * 7 = 42

So the factors of 84 are

1 A factor of everything.

2, 3, 7 The distinct prime factors.

4, 6, 14, 21 Products of two prime factors.

12, 28, 42 Products of three prime factors.

84 Product of all the prime factors.

This was a tough one! Most numbers won't have so many prime factors.

For example, consider 35. 35 = 5 * 7, so the prime factors of 35 are

5 and 7:

1 A factor of everything.

5, 7 The distinct prime factors.

35 Product of all the prime factors.

I know you'd probably like me to just give you a list of all the

factors of all the numbers from 1 to 100, but the point of thisexercise is for you to get some practice manipulating numbers. It may

seem boring, but the effort you put in now will make things much, much

easier for you later.

I hope this helps. Write back if you'd like to talk about this some

more, or if you have any other questions.

- Doctor Ian, The Math Forum


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