1. Finding The Greatest Common Factor

2. Greatest Common Factor
The greatest common factor is the largest common factor of two numbers. Factors are all of the numbers that divide into a given number equally.
The greatest common factor can be found by comparing all the factors of two numbers or by finding the prime factorization of two numbers

3. Finding Factors
Finding the factors of a number can be done by sequentially dividing numbers into the product. When the factors begin to repeat you have found all factors.

4. We are now back to 6. We have found all factors of 18.
Finding Factors
Find the factors of 18
1 goes into 18, 18 times.
2 goes into 18, 9 times.
3 goes into 18, 6times.
4 does not divide 18 evenly.
5 does not divide 18 evenly.
We are now back to 6. We have found all factors of 18.
1, 2, 3, 6 and 18

5. Finding The GCF Using Factors
Determine the greatest common factor of two numbers by finding all the factors for the two numbers then comparing them. The largest common factor of the two numbers is their greatest common factor.

6. The greatest common factor is of 36 and 24 is 12
Using Factors
Find the GCF of 36 and 24
Factors of 36: 1, 2, 3, 4, 6, 9, 12, 18 and 36
Factors of 24: 1, 2, 3, 4, 6, 8, 12 and 24
The greatest common factor is of 36 and 24 is 12

7. The greatest common factor of 15 and 100 is 5
Using Factors Again
Find the GCF of 15 and 100
Factors of 15: 1, 3, 5 and 15
Factors of 100: 1, 2, 4, 5, 10, 20, 25,
50 and 100
The greatest common factor of 15 and 100 is 5

8. Finding the Prime Factorization
A prime factorization is when a product is broken down until only prime numbers remain. Prime numbers are numbers who’s only factors and 1 and them self. A factor tree is useful in finding a prime factorization.

9. Break 18 down to only prime numbers.
Prime Factorization
Break 18 down to only prime numbers. 18 /\ 9
3 3
18 = 2 × 2 × 3

10. Finding the GCF Using Prime Factorization
To determine the greatest common factor find the prime factorization for each number. Find all of the common prime factors of the two numbers and multiply them together. Your sum is the GCF.

11. Using Prime Factorization
Find the GCF of 36 and 24
/\ /\
36= 2×2×3× = 2×2×2×3
The GCF of 36 and 24 is 2×2×3=12

12. Using Prime Factorization Again
Find the GCF of 21 and 42
/\ /\ /\
3 7
21 = 3× = 2×3×7
The GCF of 21 and 42 is 3×7 = 21

13. Greatest Common Factor
Now you know how to find the greatest common factor you if you have any other questions check out these sites about finding the GCF
Greatest Common Factor
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