Rewriting Polynomials by Factoring

Polynomials are usually written in standard form. However, depending on what they describe and what information is needed, it's sometimes useful to write them in factored form.

Method

Factoring Trinomials

There are many ways to factor an expression, such as factoring by GCF or factoring a quadratic trinomial. If a polynomial expression has a degree higher than

2,

it can be helpful to combine these methods.

Factor the trinomial

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Exercise

Factor the trinomial completely. $x^3+2x^2-3x$

Show Solution

Solution

To factor the trinomial, let's start by determining if the terms have any common factor. To do this, we'll write each term as a product of its factors.

{\color{#0000FF}{x}} \cdot x \cdot x + 2 \cdot {\color{#0000FF}{x}} \cdot x -3 \cdot {\color{#0000FF}{x}}

It can be seen that each term has an

x.

Thus, we can factor the GCF of

x

out of the entire expression.

x^3+2x^2-3x=x\left(x^2+2x-3\right)

The parentheses now contain a quadratic trinomial, which can be factored. Notice that

3

and

\text{-} 1

multiply to equal

\text{-} 3

and add to equal

2.

x^2+2x-3

RewriteRewrite

2x

as

3x-x

x^2+3x-x-3

FactorOutFactor out

x

x(x+3)-x-3

FactorOutFactor out

\text{-} 1

x(x+3)-(x+3)

FactorOutFactor out

(x+3)

(x+3)(x-1)

The expression

x^2+2x-3

can be written as the product

(x+3)(x-1).

Thus,

x^3+2x^2-3x

can be factored into

x(x+3)(x-1).

Method

Factoring by Grouping

Sometimes it's possible to factor polynomials even if its terms do not have a common factor. The polynomial

y=2x^3-x^2+6x-3

can be factored by grouping the terms.

1

Group the terms in pairs

To begin, the first two terms and the last two terms are grouped. This can be done using parentheses. $y=2x^3-x^2+6x-3 = \left(2x^3-x^2\right)+(6x-3)$

2

Factor out the GCF in each pair

In the first pair, factor out the GCF. Here, the GCF is $x^2.$ $\begin{aligned} \left(2x^3-x^2\right)\textcolor{lightgray}{+(6x-3)}\\ x^2(2x-1)\textcolor{lightgray}{+(6x-3)} \end{aligned}$ For the second pair, the GCF is $3.$ $\begin{aligned} &\textcolor{lightgray}{x^2(2x-1)+}(6x-3)\\ &\textcolor{lightgray}{x^2(2x-1)+}3(2x-1) \end{aligned}$

3

Factor out the GCF in the resulting sum

If the polynomial is factorable, this should lead to a sum of two terms with a common factor. In this case, that factor is $(2x-1).$ $\begin{aligned} {\color{#0000FF}{x^2}}&(2x-1)+{\color{#0000FF}{3}}(2x-1)\\ & \ \ (2x-1)\left({\color{#0000FF}{x^2}}+{\color{#0000FF}{3}}\right) \end{aligned}$ This means that $y=2x^3-x^2+6x-3$ can be written as $y=\left(2x-1\right)\left(x^2+3\right).$

Example

Factor the trinomial

1

2

3