Question

Вычеслить значение выражения cos(Π/18)cos(4Π/9)- sin(Π/18)since(4Π/9)

Answer 1

$\cos\frac{\pi}{18}\cos\frac{4\pi}{9}-\sin\frac{\pi}{18}\sin\frac{4\pi}{9}=\cos(\frac{\pi}{18}+\frac{4\pi}{9})=\cos\frac{9\pi}{18} =\cos\frac{\pi}{2} =0$