Answer
$$(r-20)(r^2+128)=r^3-20r^2+128r-2560$$
Explanation
Tap the blue circles to see an explanation.
| $$ \begin{aligned}(r-20)(r^2+128)& \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle1}{\textcircled {1}} } }}}r^3+128r-20r^2-2560 \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle2}{\textcircled {2}} } }}}r^3-20r^2+128r-2560\end{aligned} $$ | |
| ① | Multiply each term of $ \left( \color{blue}{r-20}\right) $ by each term in $ \left( r^2+128\right) $. $$ \left( \color{blue}{r-20}\right) \cdot \left( r^2+128\right) = r^3+128r-20r^2-2560 $$ |
| ② | Combine like terms: $$ r^3-20r^2+128r-2560 = r^3-20r^2+128r-2560 $$ |
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