Question

помогите пж..............

Answer 1

$\displaystyle cos\frac{13\pi }{8}=cos(\frac{16\pi }{8}-\frac{3\pi }{8})=cos(2\pi -\frac{3\pi }{8})=cos\frac{3\pi }{8}=\\\\=cos(\frac{4\pi }{8}-\frac{\pi }{8})=cos(\frac{\pi }{2}-\frac{\pi }{8})=sin\frac{\pi }{8}$