pt<=> 5.sin3x = 3sin5x
<=> 4.(sin5x - sin3x) = sin5x + sin3x
<=> 8.cos4x.sinx = 2.sin4x.cosx
<=> 4.cos4x.sinx = 4cos2x.cosx.sinx.cosx
<=> cos4x.sinx =cos2x.(cos^2x).sinx
=> sinx =0 => x=...
=> 2cos^2(2x) -1 = cos2x.(1-cos2x)\2
đặt cos2x =a
pt <=> 4a^2 -2 =a.(1-a)
<=> 5a^2 -a-2 =0
=> a= cos2x =..=> x=...
vậy x=...