(cosxcosy-sinxsiny)(cosxcosy+sinxsiny)=1/3
cos²xcos²y-sin²xsin²y=1/3
cos²x(1-sin²y)-(1-cos²x)sin²y=1/3
cos²x-cos²xsin²y-sin²y+cos²xsin²y=1/3
cos²x-sin²y=1/3
若cos(x+y)cos(x-y)=1/3,则cos^x-sin^y=?^为平方
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