Question

Объясните как!

Answer 1

$(- \frac{2}{ \sqrt{5}})^2+(\frac{1}{ \sqrt{5}})^2=1 \\ \frac{2^2}{( \sqrt{5})^2}+\frac{1^2}{( \sqrt{5})^2}=1 \\ \frac{4}{5}+ \frac{1}{5}=1 \\ \frac{5}{5}=1 \\ 1=1$