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ALGEBRA EASY FINDING GREATEST COMMON DENOMINATOR
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A factor of a number is a number that divides evenly into the number.

The factors of 2 are 1 and 2. The factors of 5 and 5 and 1. 2 and 5 aren't very interesting from the standpoint of factors, because they are primes. (See How to Prime Factor a Number for more about primes.) In terms of factors we can say that a number larger than 1 is a prime if its only factors are 1 and itself. Composites are more interesting with respect for factors. The factors of 12 are 1, 2, 3, 4, 6, and 12. The factors of 250 are 1, 2, 5, 10, 25, 50, 125, and 250.

Multiples

A multiple of a number is a number that you can get by multiplying it by something. 

So, the multiples of 2 are 2,4,6,8,10,12,..., and the multiples of 5 are 5,10,15,20, 25,.... The multiples are the numbers that you get when you count by the number.

Factors and Multiples

Factors and multiples are sort of opposites. If one number is a factor of another number, then the other number is a multiple of it and vice versa.
 
'A is a factor of B' means the same thing as 'B is a multiple of A'.

For example, 2 is a factor of 12 and 12 is a multiple of 2. 125 is a multiple of 5 and 5 is a factor of 125. 9 is a factor of 900 and 900 is a multiple of 9. 70 is a multiple of both 7 and 10, and both 7 and 10 are factors of 70.

It is important to not get confused which one is which. The multiples are the big ones, and the factors are the little ones. Perhaps it will help if you remember that the multiple is the result of a multiplication and the factor isn't. Or associate multiple with multitudes so that it will just sound big, so that you can remember that it is the big one, and the factor is the other one, so it must be the little one.

Common

Common in this setting means something that two or more numbers share. For example a common multiple of 4 and 6 would be a number that is both a multiple of 4 and a multiple of 6. A common factor of 4 and 6 would be a number that is a factor of both 4 and 6. 12 is a common multiple of 4 and 6, because 12 is the result of multiplying 3 times 4 and also the result of multiplying 2 times 6. 2 is a common factor of 4 and 6, because 2 divides evenly into 4, and it also divides evenly into 6, 2x2=4 and 2x3=6. It is easy to find small common factors and big common multiples, because 1 is always a common factor of any collection of numbers and if you multiply all of the numbers together you will always get a common multiple. But small common multiples and large common factors are harder to find.

Relative Primes

Two numbers are called relative prime if their only common factor is 1.

If the numbers themselves are prime they will always be relative prime, so 3 and 5 are relative prime, but they don't have to be prime to be relative prime. For example, 8 and 27 are relative prime, but definitely not primes themselves. Even a number with lots of factors like 60 can be relatively prime with respect to another number, for example 60 and 121 are relatively prime. Perhaps you can think of some other interesting examples of relative primes that are not primes.

Finding GCFs and LCMs by Guessing

Now that we know what factors and multiples and common factors and common multiples are, we are ready to understand what LCMs and GCFs are. They mean exactly what their words say. The LCM of a collection of numbers is the least or smallest number that is a common multiple of them. So if we took all of the common multiples of the numbers and lined them up and asked which one was the smallest, then that would be the least common multiple. Similarly the GCF of a collection of numbers is the greatest or largest number that is a common factor of the numbers. If we took all of the common factors of those numbers and lined them up and then took that largest one, that would be the greatest common factor. As I mentioned earlier, it is easy to find large common multiples and small common factors, so this is the more interesting task, finding a common factor that isn't any smaller than necessary and a common multiple that isn't any larger than necessary. For many collections of numbers, particularly when they aren't too large, you can do this by guessing and playing around with the numbers.

To find GCFs this way, if you don't see a large common factor right away, try looking for primes that divide the numbers evenly, and then if you find more than one of them try multiplying them together and see if that is still a common factor. For example if you have 12 and 18, you could see that both 2 and 3 are common factors, so maybe 6 will also be a common factor, and it is. Is there a larger one? Not too likely because 6 is a pretty large factor just for 12 alone, and in fact the only larger factor that 12 alone has is 12, which is clearly not a factor of 18.

For LCMs there are a couple of techniques you can use. You can always find a common multiple by multiplying the numbers, so for example 15 is a common multiple of 3 and 5, because it is 5x3 and 3x5. 12x18=216, so 216 is a common multiple of 12 and 18, since it is 18x12 and 12x18. If you are dealing with only two numbers, this will be the smallest one, so the LCM, whenever the two numbers are relatively prime, but if the numbers have a common factor larger than 1, there should be a smaller number that will work. 15 is the LCM of 3 and 5, but 216 is not the LCM of 12 and 18, but at least you know from this that the LCM for them can be no larger than 216.

On the other end of the scale, if one of your numbers is a multiple of the other, then it is easy to find the LCM, it is just the bigger number. So the LCM of 8 and 4 is 8, and the LCM of 5 and 15 is 15. For numbers like 12 and 18, you might just think of it without even knowing how you do it. Often times when I ask someone for a common multiple of two numbers without even specifying that I want the LCM, they tend to instinctively give me the LCM, so if you can do it that way, that's fine. Think of a common multiple of 12 and 18, and perhaps 36 just comes to mind, and then you make sure you can't think of a smaller one, and since 18 is the only smaller multiple of just 18, and it is not a multiple of 12, it must be the answer.

But if that doesn't work for you, there is a more systematic way you can go about the task in a problem like this. What you can do is you can take the largest of the numbers, and start listing successive multiples of it, by multiplying it by 2, 3, 4, etc., until you come up with one that is a multiple of the other numbers. So in this case it would work in just one step, since 2x18=36, and 36 is indeed a multiple of 12, but if that didn't work you could go on to 3x18, 4x18, until one of them was a multiple of 12. The reason to use the largest number when doing this is that if you do that, you won't have to go as far. In this case if you used 12 instead, you would say 2x12=24, no 24 is not a multiple of 18, 3x12=36, 36 is, so that works, and you still get the right answer but it takes one more step.

For another example, let's say we want to find the LCM of 50 and 60. Write down successive multiples of 60 until you get one that is also a multiple of 50. 60x2=120, no, 60x3=180, no, 60x4=240, no, 60x5=300, yes, we've got it, so 300 is the LCM of 50 and 60. Notice if you simply multiplied the two numbers together you would get 3000, which is much bigger. That is because 50 and 60 have a common factor of 10. Now here is something interesting. If you divide 3000 by the common factor 10, you get 300, the LCM. Actually for two numbers, this is a trick that will always work, and it might be useful in a case like this where it is easy to see what the GCF is. 10 is not just any old common factor of 50 and 60, it is the greatest common factor, and it turns out that if you divide the product by the GCF, that will always give you the LCM. If you want to use that trick, use it, but do realize that it is only a good trick for problems where the numbers are easy to multiply and the GCF is easy to find. For 12 and 18, you could do it too, 12x18=216, 216/6=36, but here is it probably easier to just multiply 18 by 2.

For finding LCMs and GCFs of larger numbers and more than 2 numbers there is a more systematic method that is also important to learn for when you later learn to do this sort of thing with algebraic expressions. This method involves prime factoring. To learn how to prime factor numbers, see my article How to Prime Factor a Number. But since too many people learn these method without understanding them, before I talk about them I want to talk a little about the relationships between the prime factorization of factors and multiples.

Factors and Multiples in Prime Factored Form

Here's the game. Let's say we have our numbers in prime factored form and we want to tell whether one number is a multiple or a factor of the other. How can we do it? Is there a good way to tell this from the prime factored form? For example, here are two numbers prime factored.

24x3x5, 3x5x7

Without multiplying out to see what the numbers are, can we tell if the first is a multiple or a factor of the second?  How about these two numbers?

3x5, 3x5x7

For these two we can see something interesting. The factorization of the first one is contained in that of the second one, which makes the first one a factor of the second one and the second one a multiple of the first one. 3x5 divides 3x5x7 evenly, namely when you divide 3x5x7 by 3x5, you get 7, and since 3x5x7=(3x5)x7, 3x5x7 can be obtained by multiplying something by 3x5, namely 7, so it is a multiple of it. In general this test will always work.
 

In prime factored form one number is a multiple of another number if its factorization contains the factorization of the other number.

In prime factored form one number is a factor of another number if its factorization is contained in the factorization of the other number.