Tap the blue circles to see an explanation.
| $$ \begin{aligned}(400-x^2)^3& \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle1}{\textcircled {1}} } }}}64000000-480000x^2+1200x^4-x^6 \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle2}{\textcircled {2}} } }}}-x^6+1200x^4-480000x^2+64000000\end{aligned} $$ | |
| ① | Find $ \left(400-x^2\right)^3 $ using formula $$ (A - B) = A^3 - 3A^2B + 3AB^2 - B^3 $$where $ A = 400 $ and $ B = x^2 $. $$ \left(400-x^2\right)^3 = 400^3-3 \cdot 400^2 \cdot x^2 + 3 \cdot 400 \cdot \left( x^2 \right)^2-\left( x^2 \right)^3 = 64000000-480000x^2+1200x^4-x^6 $$ |
| ② | Combine like terms: $$ -x^6+1200x^4-480000x^2+64000000 = -x^6+1200x^4-480000x^2+64000000 $$ |