Answer
$$(5a+b)(a-4b)=5a^2-19ab-4b^2$$
Explanation
Tap the blue circles to see an explanation.
| $$ \begin{aligned}(5a+b)(a-4b)& \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle1}{\textcircled {1}} } }}}5a^2-20ab+ab-4b^2 \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle2}{\textcircled {2}} } }}}5a^2-19ab-4b^2\end{aligned} $$ | |
| ① | Multiply each term of $ \left( \color{blue}{5a+b}\right) $ by each term in $ \left( a-4b\right) $. $$ \left( \color{blue}{5a+b}\right) \cdot \left( a-4b\right) = 5a^2-20ab+ab-4b^2 $$ |
| ② | Combine like terms: $$ 5a^2 \color{blue}{-20ab} + \color{blue}{ab} -4b^2 = 5a^2 \color{blue}{-19ab} -4b^2 $$ |
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