Cos3x/tgx = sin3x - 2sinx
cos3x*cosx/sinx = sin3x - 2sinx
cos3x*cosx = sin3x*sinx - 2*sin²x
cos3x*cosx - sin3x*sinx + 2*sin²x = 0
cos(3x+x) + 2*sin²x = 0
cos4x + 2*sin²x = 0
cos²2x - sin²2x + 2*(1-cos2x)/2 = 0
cos²2x - sin²2x + 1 - cos2x = 0
cos²2x - sin²2x + sin²2x + cos²2x - cos2x = 0
2*cos²2x - cos2x = 0
cos2x*(2*cos2x - 1) = 0
cos2x = 0
2x = pi/2 + pik
x = pi/4 + pik/2, k ∈ Z
2*cos2x - 1 = 0
cos2x = 1/2
2x = +- pi/3 + 2pik
x = +-pi/6 + pik, k ∈ Z