Question

упростите выражение cos (13π/8)

Answer 1

$cos(\frac{13\pi }{8})= cos(\frac{8\pi+5\pi }{8})=cos(\frac{8\pi }{8}+\frac{5\pi }{8})=cos(\pi + \frac{5\pi }{8})=-cos \frac{5\pi }{8}$