Question

Упростить и найти значение выражения
4sin(x/7)*cos(x/7)*cos(2x/7),при Х=7П/4

Answer 1

4sin(x/7)*cos(x/7)*cos(2x/7)=2*2*sin(x/7)*cos(x/7)*cos(2x/7)=
=2*sin(2x/7)*cos(2x/7)=sin(4x/7)
x=7π/4⇒sin(4x/7)=sin(4/7*7π/4)=sinπ=0