Question

Решите уравнение: cosx+cos2x+cos3x=0

Answer 1

cosx+cos3x+cos2x=2cos2x·cosx+cos2x=cos2x(2cosx+1)=0
1.  cos2x=0  2x=π/2+πk  x=π/4+πk/2     k∈Z
2. cosx=-1/2    x=+-2π/3+2πn  n∈Z