Question

Cos π знаменатель10 -Cos π знаменатель 20

Answer 1

$cosfrac{pi}{10}-cosfrac{pi}{20}=-2sin({frac{1}{2}(frac{pi}{10}+frac{pi}{20}))sin(frac{1}{2}(frac{pi}{10}-frac{pi}{20}))=-2sinfrac{3pi}{40}sinfrac{pi}{40}$