| $$ \begin{aligned}-\sqrt{5}\cdot(8-\sqrt{21})& \xlongequal{ }-(8\sqrt{5}-\sqrt{105}) \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle1}{\textcircled {1}} } }}}-8\sqrt{5}+\sqrt{105}\end{aligned} $$ | |
| ① | Remove the parenthesis by changing the sign of each term within them. |