sin3xsinx
=sin(x+2x)sinx
=(sinxcos2x+cosxsin2x)sinx
=sin²xcos2x+sinxcosxsin2x
=(1/2)(1-cos2x)cos2x+(1/2)sin²2x
=(1/2)cos2x-(1/2)cos²2x+(1/4)(1-cos4x)
=(1/2)cos2x-(1/4)(1+cos4x)+1/4-(1/4)cos4x
=(1/2)cos2x-1/4-(1/4)cos4x+1/4-(1/4)cos4x
=(1/2)cos2x-(1/2)cos4x
=(1/2)(cos2x-cox4x)
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