Question

Преобразовать в произведение cos11π/5 +cos3π/4

Answer 1

$cos\alpha+cos\beta = 2 cos\frac{\alpha+ \beta }{2}cos\frac{\alpha -\beta }{2}\\cos11\pi /5 +cos3\pi /4 = 2cos\frac{59\pi }{40} cos\frac{29\pi }{40}$