Answer
$$(x-9)(x+9)=x^2-81$$
Explanation
Tap the blue circles to see an explanation.
| $$ \begin{aligned}(x-9)(x+9)& \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle1}{\textcircled {1}} } }}}x^2+9x-9x-81 \xlongequal{ } \\[1 em] & \xlongequal{ }x^2+ \cancel{9x} -\cancel{9x}-81 \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle2}{\textcircled {2}} } }}}x^2-81\end{aligned} $$ | |
| ① | Multiply each term of $ \left( \color{blue}{x-9}\right) $ by each term in $ \left( x+9\right) $. $$ \left( \color{blue}{x-9}\right) \cdot \left( x+9\right) = x^2+ \cancel{9x} -\cancel{9x}-81 $$ |
| ② | Combine like terms: $$ x^2+ \, \color{blue}{ \cancel{9x}} \, \, \color{blue}{ -\cancel{9x}} \,-81 = x^2-81 $$ |
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