$$ \begin{aligned}-\sqrt{5}\cdot(3+\sqrt{3})& \xlongequal{ }-(3\sqrt{5}+\sqrt{15}) \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle1}{\textcircled {1}} } }}}-3\sqrt{5}-\sqrt{15}\end{aligned} $$ | |
① | Remove the parenthesis by changing the sign of each term within them. |