| $$ \begin{aligned}x^1+4x^2+6x^3+4x^4+x^9& \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle1}{\textcircled {1}} } }}}x^9+4x^4+6x^3+4x^2+x\end{aligned} $$ | |
| ① | Combine like terms: $$ x^9+4x^4+6x^3+4x^2+x = x^9+4x^4+6x^3+4x^2+x $$ |