Question

sin a =2/5, cos a =?

Answer 1

$sin^2a+cos^2a=1 \\ cos^2a=1-sin^2a \\ \\ cos^2a=1- \frac{4}{25} \\ \\ cos^2a= \frac{21}{25} \\ \\ cosa=\pm \frac{ \sqrt{21} }{5}$