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初中有平方差公式x^2-y^2=(x+y)(x-y),高中则遇到了sin^2 x-sin^2y=sin(x+y)sin(x-y),看似只是在平方差公式加了sin符号,经验算,该式子"竟然"也是成立的:sin^2 x-sin^2 y=(sin x+sin y)(sin x-sin y)=2 sin(x+y)/2 cos(x-y)/2.2sin(x-y)/2 cos(x+y)/2=sin(x+y)sin(x-y).