Question
Answer
$$(5i+2r\cdot20)(-5i)=-200ir+25$$
Explanation
Tap the blue circles to see an explanation.
| $$ \begin{aligned}(5i+2r\cdot20)(-5i)& \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle1}{\textcircled {1}} } }}}(5i+40r)(-5i) \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle2}{\textcircled {2}} } }}}-25i^2-200ir \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle3}{\textcircled {3}} } }}}25-200ir \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle4}{\textcircled {4}} } }}}-200ir+25\end{aligned} $$ | |
| ① | $$ 2 r \cdot 20 = 40 r $$ |
| ② | $$ \left( \color{blue}{5i+40r}\right) \cdot -5i = -25i^2-200ir $$ |
| ③ | $$ -25i^2 = -25 \cdot (-1) = 25 $$ |
| ④ | Combine like terms: $$ -200ir+25 = -200ir+25 $$ |
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