Question

перетворіть добуток cos 7Π/12 cos Π/12​

Answer 1

$Cos\frac{7\pi }{12}Cos\frac{\pi }{12}=\frac{1}{2}[Cos(\frac{7\pi }{12} +\frac{\pi }{12})+Cos(\frac{7\pi }{12}-\frac{\pi }{12})]=\frac{1}{2}(Cos\frac{2\pi }{3} +Cos\frac{\pi }{2})=\\\\=\frac{1}{2}(-\frac{1}{2}+0)=\frac{1}{2}*(-\frac{1}{2})=-\frac{1}{4}=\boxed{-0,25}$