Question

4(cos(pi/5)-cos(2pi/5)) помогите пожалуйста

Answer 1

$4cdot (cosfrac{pi }{5}-cosfrac{2pi }{5})=4cdot 2cdot sinfrac{3pi }{5}cdot sinfrac{pi}{5}=8cdot sin(frac{pi }{2}+frac{pi }{10})cdot sinfrac{pi }{5}=\\=underline {8cdot cosfrac{pi}{10}cdot sinfrac{pi}{5}}=8cdot cosfrac{pi }{10}cdot 2, sinfrac{pi }{10}, cos frac{pi }{10}=underline {16cdot sinfrac{pi }{10}cdot cos^2frac{pi }{10}}$