cau c 
sin (3x) 
= sin(2x + x) 
= sin(2x)cos(x) + cos(2x)sin(x) 
= 2sin(x)cos(x)cos(x) + (1 - 2sin² (x) ) sin(x) 
= 2sin(x)cos² (x) + sin(x) - 2sin³ (x) 
= 2sin(x) ( 1 -sin² (x) ) + sin(x) - 2sin³ (x) 
= 2sin(x) -2sin³ (x) + sin(x) - 2sin³ (x) 
= -4sin³ (x) + 3sin(x) 

sinx + sin2x - sin3x = 0 
sinx + 2sinx cosx + 4sin³x - 3sinx = 0 
2sinx cosx + 4sin³x - 2sinx = 0 
sinx cosx + 2sin³x - sinx = 0 
sinx (cosx + 2sin²x - 1) = 0 
sinx (cosx + 2(1 - cos²x) - 1) = 0 
sinx (cosx + 2 - 2cos²x - 1) = 0 
sinx (-2cos²x + cosx + 1) = 0 
sinx (2cos²x - cosx - 1) = 0 
sinx (cosx - 1)(2cosx + 1) = 0 
sinx = 0 ==> x = kπ 
cosx - 1 = 0 ==> x = kπ 
2cosx + 1 = 0 ==> x = 2π/3 + 2kπ, 5π/3 + 2kπ