Answer
$$(x+1)(x-4)(x-5)=x^3-8x^2+11x+20$$
Explanation
Tap the blue circles to see an explanation.
| $$ \begin{aligned}(x+1)(x-4)(x-5)& \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle1}{\textcircled {1}} } }}}(x^2-4x+x-4)(x-5) \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle2}{\textcircled {2}} } }}}(x^2-3x-4)(x-5) \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle3}{\textcircled {3}} } }}}x^3-5x^2-3x^2+15x-4x+20 \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle4}{\textcircled {4}} } }}}x^3-8x^2+11x+20\end{aligned} $$ | |
| ① | Multiply each term of $ \left( \color{blue}{x+1}\right) $ by each term in $ \left( x-4\right) $. $$ \left( \color{blue}{x+1}\right) \cdot \left( x-4\right) = x^2-4x+x-4 $$ |
| ② | Combine like terms: $$ x^2 \color{blue}{-4x} + \color{blue}{x} -4 = x^2 \color{blue}{-3x} -4 $$ |
| ③ | Multiply each term of $ \left( \color{blue}{x^2-3x-4}\right) $ by each term in $ \left( x-5\right) $. $$ \left( \color{blue}{x^2-3x-4}\right) \cdot \left( x-5\right) = x^3-5x^2-3x^2+15x-4x+20 $$ |
| ④ | Combine like terms: $$ x^3 \color{blue}{-5x^2} \color{blue}{-3x^2} + \color{red}{15x} \color{red}{-4x} +20 = x^3 \color{blue}{-8x^2} + \color{red}{11x} +20 $$ |
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