证明：要证2sin2y-sin2x=sin(2x+2y),即要证2sin2y=sin2x+sin(2x+2y)=2sin(2x+y)cosy（利用和差化积）即要证4sinycosy=2sin(2x+y)cosy两边消去2cosy,得2siny=sin(2x+y)也即要证2siny=sin2xcosy+sinycos2x,即siny(2-co...