# 9.11: Factoring Completely

**Basic**Created by: CK-12

**Practice**Factoring Completely

Suppose that the area of a poster hanging on your wall is 1000 square inches and that the height is 15 more inches than the width. How would you go about finding the height and width of the poster? What equation would you set up? How could you use factoring to solve the equation? In this Concept, you'll learn how to factor an expression completely in order to solve real-world problems such as this one.

### Guidance

We say that a polynomial is **factored completely** when we factor as much as we can and we are unable to factor any more. Here are some suggestions that you should follow to make sure that you factor completely.

\begin{align*}\checkmark\end{align*}

\begin{align*}\checkmark\end{align*} Identify special products such as the difference of squares or the square of a binomial. Factor according to their formulas.

\begin{align*}\checkmark\end{align*} If there are no special products, factor using the methods we learned in the previous Concepts.

\begin{align*}\checkmark\end{align*} Look at each factor and see if any of these can be factored further.

#### Example A

*Factor the following polynomials completely.*

(a) \begin{align*}2x^2-8\end{align*}

(b) \begin{align*}x^3+6x^2+9x\end{align*}

**Solution:**

(a) Look for the common monomial factor: \begin{align*}2x^2-8=2(x^2-4)\end{align*}. Recognize \begin{align*}x^2-4\end{align*} as a difference of squares. We factor as follows: \begin{align*}2(x^2-4)=2(x+2)(x-2)\end{align*}. If we look at each factor we see that we can't factor anything else. The answer is \begin{align*}2(x+2)(x-2)\end{align*}.

(b) Recognize this as a perfect square and factor as \begin{align*}x(x+3)^2\end{align*}. If we look at each factor we see that we can't factor anything else. The answer is \begin{align*}x(x+3)^2\end{align*}.

**Factoring Common Binomials**

The first step in the factoring process is often factoring the common monomials from a polynomial. Sometimes polynomials have common terms that are binomials. For example, consider the following expression:

\begin{align*}x(3x+2)-5(3x+2)\end{align*}

You can see that the term \begin{align*}(3x+2)\end{align*} appears in both terms of the polynomial. This common term can be factored by writing it in front of a set of parentheses. Inside the parentheses, we write all the terms that are left over when we divide them by the common factor.

\begin{align*}(3x+2)(x-5)\end{align*}

This expression is now completely factored. Let’s look at some more examples.

#### Example B

*Factor* \begin{align*}3x(x-1)+4(x-1)\end{align*}.

**Solution:** \begin{align*}3x(x-1)+4(x-1)\end{align*} has a common binomial of \begin{align*}(x-1)\end{align*}.

When we factor the common binomial, we get \begin{align*}(x-1)(3x+4)\end{align*}.

**Solving Real-World Problems Using Polynomial Equations**

Now that we know most of the factoring strategies for quadratic polynomials, we can see how these methods apply to solving real-world problems.

#### Example C

*The product of two positive numbers is 60. Find the two numbers if one of the numbers is 4 more than the other.*

**Solution:** \begin{align*}x=\end{align*} one of the numbers and \begin{align*}x+4\end{align*} equals the other number. The product of these two numbers equals 60. We can write the equation as follows:

\begin{align*}x(x+4)=60\end{align*}

Write the polynomial in standard form.

\begin{align*}x^2+4x&=60\\ x^2+4x-60&=0\end{align*}

**Factor:** \begin{align*}-60=6 \times(-10)\end{align*} *and* \begin{align*}6+(-10)=-4\end{align*}

\begin{align*}-60=-6 \times 10\end{align*} *and* \begin{align*}-6+10=4\end{align*} This is the correct choice.

The expression factors as \begin{align*}(x+10)(x-6)=0\end{align*}.

**Solve:**

\begin{align*}x+10=0 && x-6& =0\\ \text{or} \\ x=-10 && x& =6\end{align*}

Since we are looking for positive numbers, the answer must be positive.

\begin{align*}x=6\end{align*} for one number, and \begin{align*}x+4=10\end{align*} for the other number.

**Check:** \begin{align*}6 \cdot 10=60\end{align*} so the answer checks.

### Guided Practice

*Factor completely: \begin{align*}24x^3-28x^2+8x\end{align*}.*

**Solution:**

First, notice that each term has \begin{align*}4x\end{align*} as a factor. Start by factoring out \begin{align*}4x\end{align*}:

\begin{align*}24x^3-28x^2+8x=4x(6x^2-7x+2)\end{align*}

Next, factor the trinomial in the parenthesis. Since \begin{align*}a\neq 1\end{align*} find \begin{align*}a\cdot c\end{align*}: \begin{align*} 6\cdot 2=12\end{align*}. Find the factors of 12 that add up to -7. Since 12 is positive and -7 is negative, the two factors should be negative:

\begin{align*} 12&=-1\cdot -12 && and && -1+-12=-13\\ 12&=-2\cdot -6 && and && -2+-6=-8\\ 12&=-3\cdot -4 && and && -3+-4=-7\end{align*}

Rewrite the trinomial using \begin{align*}-7x=-3x-4x\end{align*}, and then factor by grouping:

\begin{align*}6x^2-7x+2=6x^2-3x-4x+2=3x(2x-1)-2(2x-1)=(3x-2)(2x-1)\end{align*}

The final factored answer is:

\begin{align*}4x(3x-2)(2x-1)\end{align*}

### Practice

Sample explanations for some of the practice exercises below are available by viewing the following video. Note that there is not always a match between the number of the practice exercise in the video and the number of the practice exercise listed in the following exercise set. However, the practice exercise is the same in both. CK-12 Basic Algebra: Factor by Grouping and Factoring Completely (13:57)

Factor completely.

- \begin{align*}2x^2+16x+30\end{align*}
- \begin{align*}12c^2-75\end{align*}
- \begin{align*}-x^3+17x^2-70x\end{align*}
- \begin{align*}6x^2-600\end{align*}
- \begin{align*}-5t^2-20t-20\end{align*}
- \begin{align*}6x^2+18x-24\end{align*}
- \begin{align*}-n^2+10n-21\end{align*}
- \begin{align*}2a^2-14a-16\end{align*}
- \begin{align*}2x^2-512\end{align*}
- \begin{align*}12x^3+12x^2+3x\end{align*}

Solve the following application problems.

- One leg of a right triangle is seven feet longer than the other leg. The hypotenuse is 13 feet. Find the dimensions of the right triangle.
- A rectangle has sides of \begin{align*}x+2\end{align*} and \begin{align*}x-1\end{align*}. What value of \begin{align*}x\end{align*} gives an area of 108?
- The product of two positive numbers is 120. Find the two numbers if one number is seven more than the other.
- Framing Warehouse offers a picture-framing service. The cost for framing a picture is made up of two parts. The cost of glass is $1 per square foot. The cost of the frame is $2 per linear foot. If the frame is a square, what size picture can you get framed for $20.00?

**Mixed Review**

- The area of a square varies directly with its side length.
- Write the general variation equation to model this sentence.
- If the area is 16 square feet when the side length is 4 feet, find the area when \begin{align*}s=1.5 \ feet\end{align*}.

- The
**surface area**is the total amount of surface of a three-dimensional figure. The formula for the surface area of a cylinder is \begin{align*}SA=2 \pi r^2+2 \pi rh\end{align*}, where \begin{align*}r=radius\end{align*} and \begin{align*}h=height \ of \ the \ cylinder\end{align*}. Determine the surface area of a soup can with a radius of 2 inches and a height of 5.5 inches. - Factor \begin{align*}25g^2-36\end{align*}. Solve this polynomial when it equals zero.
- What is the greatest common factor of \begin{align*}343r^3 t, 21t^4\end{align*}, and \begin{align*}63rt^5\end{align*}?
- Discounts to the hockey game are given to groups with more than 12 people.
- Graph this solution on a number line.
- What is the domain of this situation?
- Will a church group with 12 members receive a discount?

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Here you'll learn how to apply all the skills you have acquired so far in order to completely factor an expression.