Question

Умоляю помогите решить((
cos7П/8*cos3П/8+sin7П/8*sin3П/8

Answer 1

$Cos frac{7 pi }{8}Cos frac{3 pi }{8} +Sin frac{7 pi }{8}Sin frac{3 pi }{8} =Cos( frac{7 pi }{8} - frac{3 pi }{8}) = Cos frac{4 pi }{8}= Cos frac{ pi }{2} =0$