KarinaKoval098 Думаю этого будет достаточно.
8__________
2cos(2x) - 1 = (2cos(2x) - 1)tg(x) 

2cos(2x) - 1 = 0 --> x = ± π/6 + πk 

tg(x) = 1 --> x = π/4 + πk 

Ответ: { ±π/6 + πk; π/4 + πk\ k - целое}.
4__________
sin3x - sin x=2sin3x - x/2cos3x + x/2 = 2sinxcos2x
2sinxcos2x+ cos2x = 1
2sinxcos2x=1-cos^2x+ sin^2x
2sinxcos2x=0
sinx = 0cos2x =0
1)sinx = 0
x = ПИ*П
2)cos2x = 0
2x = pi/2+pn
x = pi/4+pn/2