| $$ \begin{aligned}\sqrt{20.8}& \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle1}{\textcircled {1}} } }}}2\sqrt{5}\end{aligned} $$ | |
| ① | $$ \sqrt{20} =
\sqrt{ 2 ^2 \cdot 5 } =
\sqrt{ 2 ^2 } \, \sqrt{ 5 } =
2 \sqrt{ 5 }$$ |