Answer
$$(8x+9)(8x-9)=64x^2-81$$
Explanation
Tap the blue circles to see an explanation.
| $$ \begin{aligned}(8x+9)(8x-9)& \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle1}{\textcircled {1}} } }}}64x^2-72x+72x-81 \xlongequal{ } \\[1 em] & \xlongequal{ }64x^2 -\cancel{72x}+ \cancel{72x}-81 \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle2}{\textcircled {2}} } }}}64x^2-81\end{aligned} $$ | |
| ① | Multiply each term of $ \left( \color{blue}{8x+9}\right) $ by each term in $ \left( 8x-9\right) $. $$ \left( \color{blue}{8x+9}\right) \cdot \left( 8x-9\right) = 64x^2 -\cancel{72x}+ \cancel{72x}-81 $$ |
| ② | Combine like terms: $$ 64x^2 \, \color{blue}{ -\cancel{72x}} \,+ \, \color{blue}{ \cancel{72x}} \,-81 = 64x^2-81 $$ |
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