Solution: The GCF of 81 and 9 is 9
Methods
How to find the GCF of 81 and 9 using Prime Factorization
One way to find the GCF of 81 and 9 is to compare the prime factorization of each number. To find the prime factorization, you can follow the instructions for each number here:
What are the Factors of 81?
What are the Factors of 9?
Here is the prime factorization of 81:
3
4
3^4
3 4
And this is the prime factorization of 9:
3
2
3^2
3 2
When you compare the prime factorization of these two numbers, you can see that there are matching prime factors. You can now find the Greatest Common Factor of 81 and 9 by multiplying all the matching prime factors to get a GCF of 81 and 9 as 9:
Thus, the GCF of 81 and 9 is: 9
How to Find the GCF of 81 and 9 by Listing All Common Factors
The first step to this method of finding the Greatest Common Factor of 81 and 9 is to find and list all the factors of each number. Again, you can see how this is done by looking at the “Factors of” articles that are linked to above.
Let’s take a look at the factors for each of these numbers, 81 and 9:
Factors of 81: 1, 3, 9, 27, 81
Factors of 9: 1, 3, 9
When you compare the two lists of factors, you can see that the common factor(s) are 1, 3, 9. Since 9 is the largest of these common factors, the GCF of 81 and 9 would be 9.
Find the GCF of Other Number Pairs
Want more practice? Try some of these other GCF problems:
What is the GCF of 43 and 82?
What is the GCF of 92 and 124?
What is the GCF of 133 and 117?
What is the GCF of 63 and 135?
What is the GCF of 70 and 100?