Question

cos3x+cos9x=0

Answer 1

cos3x+cos9x=0, 2cos6x*cos3x=0, cos6x =0 или cos3x=0, 6х=π/2 +2πк, 3х=π/2 + 2πк, х = π/12 + πк/3, х = π/6 + 2πк/3. Ответ: х = π/12 + πк/3, х = π/6 + 2πк/3