# Factoring Out the Greatest Common Factor (GCF)

Now that we know how to find the greatest common factor of monomials, we can take the first step in factoring a polynomial.

If I give you a polynomial and ask you to factor it, the first thing you need to do is look at all of the terms and find their greatest common factor. (If there is one.) Once you've found it, you can write every term as a product of the GCF and another monomial and then factor the GCF out. Here's how it works in practice:

Factor: |
$3x^3y + 6x^2y^2$ | |

1. Find the greatest common factor of the polynomial's terms. The polynomial's terms are $3x^3y$ and $6x^2y^2$. Applying the techniques that we learned in the "Common Factors" lesson, we can see that the greatest common factor of those monomials is $3x^2y$. |
$3x^2y$ | |

2. Rewrite each of the terms using the greatest common factor. Take a close look at how this step works - it's the key one in the process. Notice how $3x^2y \cdot x = 3x^3y$ which is the first term in the original polynomial and $3x^2y \cdot 2y = 6x^2y^2$ which is the original second term. |
$3x^2y \cdot x + 3x^2y \cdot 2y$ | |

3. Factor the GCF out of every term. This is going to give us our final answer. |
$3x^2y(x + 2)$ |

# Example 1

**Simplify 6 x^{3} + 2x^{2} - 2x.**

The three terms we have to look at are 6*x*^{3}, 2*x*^{2} and 2*x*. (For this part of the discussion we don't need to worry about the negative sign.) Applying the techniques of the "Common Factors" lession, the greatest common factor of those three terms is 2*x*.

This brings us to step two. We need to rewrite each of the three terms as something times the greatest common facor. That would look like:

2*x* · 3*x*^{2} + 2*x* · *x* - 2*x* · 1

Notice how I put a 1 on the last term. A term is never going to go away because of factoring out the greatest common factor. There has to be something left behind, even if it's just a 1.

Now I'm going to factor out the 2*x* and leave everything else behind, including the negative sign.

2*x* (3*x*^{2} + *x* - 1)

# Example 3

**Simplify 8 m^{2}n^{2}o - 4mn^{2}o + 20m^{2}n^{3}o^{3}.**

This problem follows the same pattern we've seen in previous examples. First we find the greatest common factor of 8*m*^{2}*n*^{2}*o*, 4*mn*^{2}*o* and 20*m*^{2}*n*^{3}*o*^{3}. Using the techniques of "Common Factors" we can see that the greatest common factor is 4*mn*^{2}*o*. If I rewrite all the terms of the original polynomial in terms of 4*mn*^{2}*o*, I get:

4*mn*^{2}*o* · 2*m* - 4*mn*^{2}*o* · 1 + 4*mn*^{2}*o* · 5*mno*^{2}

Now I'll factor out the 4*mn*^{2}*o* and be left with the factored version.

4*mn*^{2}*o*(2*m* - 1 + 5*mno*^{2})

# Example 2

**Simplify 12 xy^{3} - 16x^{4}y^{3}.**

Applying the techniques of the "Common Factors" lesson, we can see that the greatest common factor of 12*xy*^{3} and 16*x*^{4}*y*^{3} is 4*xy*^{3}. If I rewrite each of the terms in terms of 4*xy*^{3} it looks like:

4*xy*^{3} · 3 - 4*xy*^{3} · 4*x*^{3}

Now if we factor out the 4*xy*^{3} we get the final factorization.

4*xy*^{3} (3 - 4*x*^{3})

# Example 4

**Factor -81 ab^{2} - 12ab^{2}c - 3bc^{3}.**

This one starts out much like the others but it has a little twist at the end. First, we need the greatest common factor of 81*ab*^{2}, 12*ab*^{2}*c* and 3*bc*^{3}. Applying the techniques of the "Common Factors" lesson, we can see that that's 3*b*. Rewriting each of the terms in the original expression in terms of 3*b* gives us:

-3*b* · 27*ab* - 3*b* · 4*abc* - 3*b* · *c*^{3}

Notice how, once again, I left all the negative signs where they were. Now I'm going to factor out all of the 3*b*'s to get

3*b*(-27*ab* - 4*abc* - *c*^{3})

That's a perfectly good factorization and there'd be nothing wrong with stopping there. However, if you'll look closely, you'll see that all three of the terms have a negative sign. Whenever all three of the terms have something, you should always ask yourself if it's possible to factor it out. In this case we could simplify the expression a little further by pulling out the negative sign. We'll follow the same pattern here that we did with the 3*b*. First, I'll write every term in terms of -1. Pay careful attention to how I handle the plus signs.

3*b*(-1 · 27*ab* + (-1) · 4*abc* + (-1) · *c*^{3})

Now I can factor the negative one out to get:

(-1) · 3*b*(27*ab* + 4*abc* + *c*^{3})

-3*b*(27*ab* + 4*abc* + *c*^{3})

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## Dynamic Tutorial - Factoring Out the Greatest Common Factor

**Directions:** This solution has 6 steps. To see a description of each step click on the boxes on the left side below. To see the calculations, click on the corresponding box on the right side. Try working out the solution yourself and use the descriptions if you need a hint and the calculations to check your solution.