Question

COS 21П/4 Вычислить

Answer 1

$displaystyle cos{(frac{21pi }4 )} =cos{(frac{20pi +pi }4 )} =\\=cos{(5pi +frac{pi }4 )} =cos{(2cdot 2pi +pi +frac{pi }4 )} =\\=cos{(pi +frac{pi }4 )} =-cos{frac{pi }4 } =boxed{-frac{sqrt2 }2 }$