$$ \begin{aligned}-\sqrt{3}\cdot(-9+\sqrt{5})& \xlongequal{ }-(-9\sqrt{3}+\sqrt{15}) \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle1}{\textcircled {1}} } }}}9\sqrt{3}-\sqrt{15}\end{aligned} $$ | |
① | Remove the parenthesis by changing the sign of each term within them. |