
Factoring Numbers "Factors" are the numbers you multiply to get another number. For instance, the factors of 15 are 3 and 5, because 3×5 = 15. Some numbers have more than one factorization (more than one way of being factored). For instance, 12 can be factored as 1×12, 2×6, or 3×4. A number that can only be factored as 1 times itself is called "prime". The first few primes are 2, 3, 5, 7, 11, and 13. The number 1 is not regarded as a prime, and is usually not included in factorizations, because 1 goes into everything. (The number 1 is a bit boring in this context, so it gets ignored.) You most often want to find the "prime factorization" of a number: the list of all the primenumber factors of a given number. The prime factorization does not include 1, but does include every copy of every prime factor. For instance, the prime factorization of 8 is 2×2×2, not just "2". Yes, 2 is the only factor, but you need three copies of it to multiply back to 8, so the prime factorization includes all three copies.
On the other hand, the
prime factorization includes ONLY the prime factors, not any products
of those factors. For instance, even though 2×2 = 4, and even though 4 is a divisor of 8, 4 is NOT in the
PRIME factorization of 8.
That is because 8 does NOT equal 2×2×2×4!
This accidental overduplication of factors is another reason why the
prime factorization is often best: it avoids counting any factor too many
times. Suppose that you need to find the prime factorization of 24.
Sometimes a student will just list all the divisors of 24:
1, 2, 3, 4, 6, 8, 12, and 24.
Then the student will do something like make the product of all these
divisors: 1×2×3×4×6×8×12×24.
But this equals 331776,
not 24.
So it's best to stick to the prime factorization, even if the problem
doesn't require it, in order to avoid either omitting a factor or else
overduplicating one. In the case of 24, you can find the prime factorization by taking 24 and dividing it by the smallest prime number that goes into 24: 24 ÷ 2 = 12. (Actually, the "smallest" part is not as important as the "prime" part; the "smallest" part is mostly to make your work easier, because dividing by smaller numbers is simpler.) Now divide out the smallest number that goes into 12: 12 ÷ 2 = 6. Now divide out the smallest number that goes into 6: 6 ÷ 2 = 3. Since 3 is prime, you're done factoring, and the prime factorization is 2×2×2×3.
The nice thing about this upsidedown division is that, when you're done, the prime factorization is the product of all the numbers around the outside. The factors are circled in red above. By the way, this upsidedown division is something that should probably be done on scratchpaper, and not handed in as part of your homework. Copyright © Elizabeth Stapel 20002011 All Rights Reserved
Some texts prefer that answers such as this be written using exponential notation, in which case the final answer would be written as 2×3×5^{2}×7. You can do the repeated division "rightside up", too, if you prefer. The process works the same way, but the division is reversed in orientation. The above problem would be worked out like this:
I'll do the repeated division: 1092 = 2 × 2 × 3 × 7 × 13 This answer might also be written as 2^{2}×3×7×13. By the way, there are some divisibility rules that can help you find the numbers to divide by. There are many divisibility rules, but the simplest to use are these:
Of course, if the number is divisible twice by 2, then it's divisible by 4; if it's divisible by 2 and by 3, then it's divisible by 6; and if it's divisible twice by 3 (or if the sum of the digits is divisible by 9), then it's divisible by 9. But since you're finding the prime factorization, you don't really care about these nonprime divisibility rules. There is a rule for divisibility by 7, but it's complicated enough that it's probably easier to just do the division on your calculator and see if it comes out even. If you run out of small primes and you're not done factoring, then keep trying bigger and bigger primes (11, 13, 17, 19, 23, etc) until you find something that works — or until you reach primes whose squares are bigger than what you're dividing into. Why? If your prime doesn't divide in, then the only potential divisors are bigger primes. Since the square of your prime is bigger than the number, then a bigger prime must have as its remainder a smaller number than your prime. The only smaller number left, since all the smaller primes have been eliminated, is 1. So the number left must be prime, and you're done. You can use the Mathway widget below to practice finding the prime factorization. Try the entered exercise, or type in your own exercise. Then click "Answer" to compare your answer to Mathway's.
(Clicking on "View Steps" on the widget's answer screen will take you to the Mathway site, where you can register for a free sevenday trial of the software.)


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