x=x/2+x/2=(x/2+y/2)+(x/2-y/2)y=(x/2+y/2)-(x/2-y/2)所以左边=sin(x/2+y/2)cos(x/2-y/2)+cos(x/2+y/2)sin(x/2-y/2)-sin(x/2+y/2)cos(x/2-y/2)+cos(x/2+y/2)sin(x/2-y/2)=2cos(x/2+y/2)sin(x/2-y/2)
sinx-siny=2cos（x+y/2）sin（x-y/2）
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