Sinx+sin2x+sin3x+sin4x=0
(sinx+sin4x)+(sin2x+sin3x)=0 2sin(5x/2)cos(3x/2)+2sin(5x/2)cos(x/2)=0 2cos(x/2)*(sin(5x/2)+sin(3x/2))=0 4cos(x/2)*sin(2x)cos(x/2)=0 4cos²(x/2)*sin(2x)=0 cos(x/2)=0 x/2=π/2+πk x=π+2πk,k∈z sin2x=0⇒2x=πk⇒x=πk/2,k∈z