sin(2x) = 2sin(x).cos(x)
sin(3x) = 3sin(x)-4sin³(x)

sin(x)-sin(2*x)+sin(3*x)=0
sin(x)-2sin(x).cos(x)+ 3sin(x)-4sin³(x)= 0

sin(x).[1-2cos(x)+3-4sin²(x)] = 0
sin(x).[4-2cos(x)-4sin²(x)]= 0
2.sin(x).[2-cos(x)-2sin²(x)] = 0
2.sin(x).[2-cos(x)-2(1-cos²(x))] = 0
2.sin(x).[2-cos(x)-2 + 2cos²(x)] = 0
2.sin(x).[-cos(x) + 2cos²(x)] = 0
2.sin(x).cos(x).(2cos(x) -1) = 0

C'est maintenant facile ...

Sauf distraction.