Question

Преобразовать в сумму cos7Π/12 *cosΠ/12

Answer 1

Cos$\frac{7pi}{12}$*cos$\frac{pi}{12}$=$\frac{cos( \frac{7pi}{12}+ \frac{pi}{12} )+cos( \frac{7pi}{12}- \frac{pi}{12} )}{2}$=$\frac{cos \frac{8pi}{12}+cos \frac{6pi}{12} }{2}$=$\frac{cos \frac{2pi}{3}+cos \frac{pi}{2} }{2}$=$\frac{-0.5+0}{2}$=-0.25