Answer:
First off, if you're in a rush, here's the answer to the question "what is the GCF of 9 and 45?":
GCF of 9 and 45 = 9
What is the Greatest Common Factor?
Put simply, the GCF of a set of whole numbers is the largest positive integer (i.e whole number and not a decimal) that divides evenly into all of the numbers in the set. It's also commonly known as:
Greatest Common Denominator (GCD)
Highest Common Factor (HCF)
Greatest Common Divisor (GCD)
There are a number of different ways to calculate the GCF of a set of numbers depending how many numbers you have and how large they are.
For smaller numbers you can simply look at the factors or multiples for each number and find the greatest common multiple of them.
For 9 and 45 those factors look like this:
Factors for 9: 1, 3, and 9
Factors for 45: 1, 3, 5, 9, 15, and 45
As you can see when you list out the factors of each number, 9 is the greatest number that 9 and 45 divides into.
Prime Factors
As the numbers get larger, or you want to compare multiple numbers at the same time to find the GCF, you can see how listing out all of the factors would become too much. To fix this, you can use prime factors.
List out all of the prime factors for each number:
Prime Factors for 9: 3 and 3
Prime Factors for 45: 3, 3, and 5
Now that we have the list of prime factors, we need to find any which are common for each number.
Looking at the occurences of common prime factors in 9 and 45 we can see that the commonly occuring prime factors are 3 and 3.
To calculate the prime factor, we multiply these numbers together:
GCF = 3 x 3 = 9
Find the GCF Using Euclid's Algorithm
The final method for calculating the GCF of 9 and 45 is to use Euclid's algorithm. This is a more complicated way of calculating the greatest common factor and is really only used by GCD calculators.
Hopefully you've learned a little math today and understand how to calculate the GCD of numbers. Grab a pencil and paper and give it a try for yourself. (or just use our GCD calculator - we won't tell anyone)
