# 9.3: Special Products of Polynomials

**At Grade**Created by: CK-12

## Learning Objectives

At the end of this lesson, students will be able to:

- Find the square of a binomial.
- Find the product of binomials using sum and difference formula.
- Solve problems using special products of polynomials.

## Vocabulary

Terms introduced in this lesson:

- second-degree trinomial
- square of a binomial, binomial square
- sum and difference of terms
- difference of squares

## Teaching Strategies and Tips

In this lesson, students learn about special products of binomials.

- Have students learn to recognize the basic patterns.
- In classroom examples, use colors to denote the numbers playing the role of \begin{align*}a\end{align*} and \begin{align*}b\end{align*} in the formulas.

In the special formulas, point out that \begin{align*}b\end{align*} is considered positive; the sign does *not* go with the term.

Example:

*Square the binomial.*

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

Solution:

The minus sign tells us to use \begin{align*}(a - b)^2 = a^2 - 2ab + b^2\end{align*}. Setting \begin{align*}a = x\end{align*} and \begin{align*}b = 3\end{align*} (and not \begin{align*}-3\end{align*}),

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

## Error Troubleshooting

General Tip: Students commit a very common error when they write, for example, \begin{align*}(x + y)^2 = x^2 + y^2\end{align*}; that is, they distribute the exponent over addition instead of multiplication.

- The power rule for products does not apply to sums or differences within the parentheses. In general, \begin{align*}(x^n + y^m)^p \neq x^{np} + y^{mp}\end{align*}.
- Students can learn to avoid this mistake by recalling the exponent definition. Therefore, a polynomial raised to an exponent means that the polynomial is multiplied by itself as many times as the exponent indicates. For example:

\begin{align*}(x + y)^2 = (x + y)(x + y)\end{align*}

- See
*Review Questions*1-4, especially Problem 4.

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