# 4.2 Factoring the Greatest Common Factor of a Polynomial

When we study fractions, we learn that the greatest common factor (GCF) of two numbers is the largest number that divides evenly into both numbers. For instance, $4$ is the GCF of $16$ and $20$ because it is the largest number that divides evenly into both $16$ and $20$ The GCF of polynomials works the same way: $4x$ is the GCF of $16x$ and $20x^2$ because it is the largest polynomial that divides evenly into both $16x$ and $20x^2$.

When factoring a polynomial expression, our first step should be to check for a GCF. Look for the GCF of the coefficients, and then look for the GCF of the variables.

## Greatest Common Factor

The greatest common factor (GCF) of polynomials is the largest polynomial that divides evenly into the polynomials.

## How To

Given a polynomial expression, factor out the greatest common factor.

1. Identify the GCF of the coefficients.
2. Identify the GCF of the variables.
3. Combine to find the GCF of the expression.
4. Determine what the GCF needs to be multiplied by to obtain each term in the expression.
5. Write the factored expression as the product of the GCF and the sum of the terms we need to multiply by.

## Example 1: Factoring the Greatest Common Factor

1. Factor $6x^3y^3+45x^2y^2+21xy$.

### Analysis

After factoring, we can check our work by multiplying. Use the distributive property to confirm that $\left(3xy\right)\left(2x^2y^2+15xy+7\right)=6x^3y^3+45x^2y^2+21xy$.

1. Factor $x\left(b^2-a\right)+6\left(b^2-a\right)$ by pulling out the GCF.