Question
Answer
$$-\sqrt{21}\cdot(2-\sqrt{5})=-2\sqrt{21}+\sqrt{105}$$
Explanation
| $$ \begin{aligned}-\sqrt{21}\cdot(2-\sqrt{5})& \xlongequal{ }-(2\sqrt{21}-\sqrt{105}) \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle1}{\textcircled {1}} } }}}-2\sqrt{21}+\sqrt{105}\end{aligned} $$ | |
| ① | Remove the parenthesis by changing the sign of each term within them. |
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