Question
Answer
$$(6+i)^3=107i+198$$
Explanation
Tap the blue circles to see an explanation.
| $$ \begin{aligned}(6+i)^3& \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle1}{\textcircled {1}} } }}}216+108i+18i^2+i^3 \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle2}{\textcircled {2}} \htmlClass{explanationCircle explanationCircle3}{\textcircled {3}} } }}}216+108i-18-i \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle4}{\textcircled {4}} } }}}107i+198\end{aligned} $$ | |
| ① | Find $ \left(6+i\right)^3 $ using formula $$ (A + B) = A^3 + 3A^2B + 3AB^2 + B^3 $$where $ A = 6 $ and $ B = i $. $$ \left(6+i\right)^3 = 6^3+3 \cdot 6^2 \cdot i + 3 \cdot 6 \cdot i^2+i^3 = 216+108i+18i^2+i^3 $$ |
| ② | $$ 18i^2 = 18 \cdot (-1) = -18 $$ |
| ③ | $$ i^3 = \color{blue}{i^2} \cdot i =
( \color{blue}{-1}) \cdot i =
- \, i $$ |
| ④ | Combine like terms: $$ \color{blue}{108i} \color{blue}{-i} \color{red}{-18} + \color{red}{216} = \color{blue}{107i} + \color{red}{198} $$ |
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