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 Divisibility Greatest common factor (GCF) There are differents way to find the GCF of numbers. Look at them and choose the one you prefer!!! List the factors First list all of the factors of each number, then list the common factors and choose the largest one. Find the GCF of 12 and 36. The factors of 12 are 1, 2, 3, 4, 6 and 12. The factors of 36 are 1, 2, 3, 4, 6, 9, 12, 18 and 36. The common factors of 12 and 36 are 1, 2, 3, 4, 6 and 12 Although the numbers in bold are all common factors of both 12 and 36, 12 is the greatest common factor. Use prime factorization To find the GCF of a set of numbers, you must factor each of the numbers into primes. Then for each different prime number in all of the factorizations, do the following... 1. Count the number of times each prime number appears in all the factorizations. 2. For each prime number, take the lowest of these counts and write the result. 3. The greatest common factor is the product of all the prime numbers written down. Example: GCF (4,6)=2, because 4=2·2 and 6=2·3, so GCF(4,6)=2 If GCF(a,b)=1, is said that a and b are relative primes Find GCF(72,90,120) 1. Determine the prime factorization of each number: 72=23·32 90=2·32·5 120=23·3·5 2. Take the prime numbers that appears in all the factorizations. (Remember taking the lowest number of times they appear) Prime numbers selected: 2 and 3 3. GCF(72,90,120)=2·3=6 Find the GCF of 21, 91 and 196 GCF(21,91,196)=