3) cos 3x = sin2x
<=> cos 3x = cos ( π/2 - 2x)
<=> 3x = π/2 - 2x + k2π
Hoặc 3x = - π/2 + 2x + k2π
<=> x = π/10 + k2π/5
Hoặc x =- π/2 + k2π
Vậy...
4) sin (x + 120°) + cos2x = 0
<=> sin (x + 120°) = - cos2x
<=> sin (x + 120°) = sin ( 2x - 90°)
<=> x + 120° = 2x - 90° + 360°k
Hoặc x + 120° = 90° - 2x + 90° + 360°k
<=> x = 30° - 360°k
Hoặc x = 20° + 120°k
Vậy...

5) cos ( 2x + π/3) + cos ( x - π/3) = 0
<=> cos ( 2x + π/3) = - cos ( x - π/3)
<=> 2x + π/3 = - x + π/3 + k2π
Hoặc 2x + π/3 = x - π/3 + k2π
<=> x = k2π/3
Hoặc x = - 2π/3 + k2π
Vậy...
6) sin 3x + sin ( π/4 - x/2) =0
<=> sin ( π/4 - x/2) = - sin 3x
<=> π/4 - x/2 = - 3x + k2π
Hoặc π/4 - x/2 = π + 3x + k2π
<=> x = π/10 + 5kπ
Hoặc x = - 3π/14 - 7πk
Vậy...