Question

решить cos p/7 + cos 6p/7

Answer 1

$Cos\frac{\pi }{7}+Cos\frac{6\pi }{7}=2Cos\frac{\frac{\pi }{7}+\frac{6\pi }{7}}{2}Cos\frac{\frac{\pi }{7}-\frac{6\pi }{7}}{2}=2Cos\frac{\pi }{2} *Cos\frac{5\pi }{14}=2*0*Cos\frac{5\pi }{14}=\boxed0$