Question

Определите знак выражения
cos$\frac{5\pi }{7}$ + cos$\frac{3\pi }{4}$

Answer 1

cos5π/7+cos3π/4 ??
cos(π-2π/7)+cos(π-π/4)=-cos(2π/7)-cos(π/4)=

-(cos2π/7+cosπ/4)<0

(cos(2π/7)>0;cos(π/4)>0)

(cos5π/7+cos3π/4)<0