pi=>sinx.sin2x + sin3x = 6.cos^3x
<=>2sin²x.cosx + 3sinx - 4sin^3x = 6cosx.(1 - sin²x)
<=>8sin²x.cosx -4sin^3x + 3sinx-6cosx=0
<=>4sin²x(2cosx - sinx) - 3(2cosx - sinx) = 0
<=>(2cosx - sinx)(4sin²x - 3) = 0
PT <=> sin^3x (1 - 2sin^2x) = cos^3x (2cos^2x - 1)
<=> cos 2x (sin^3x - cos^3x) = 0
<=> cos 2x (sin x - cos x)(1 + sin x cos x) = 0
<=> cos 2x = 0 hoặc tan x = 1 hoặc sin 2x = -1/2