(cosx)^2-(cosy)^2=1/3求sin(x+y)*sin(x-y).
问题描述：
(cosx)^2-(cosy)^2=1/3求sin(x+y)*sin(x-y).
答
cos²x-cos²y=1/3sin(x+y)sin(x-y)=(sinxcosy+cosxsiny)(sinxcosy-cosxsiny)=sin²xcos²y-cos²xsin²y=(1-cos²x)cos²y-cos²x(1-cos²y)=cos²y-cos²xcos...第三行怎么得到的sin(a+b)=sinacosb+cosasinbsin(a-b)=sinacosb-cosasinbsin(x+y)sin(x-y)=(sinxcosy+cosxsiny)(sinxcosy-cosxsiny) ///这个就是普通的划开=sin²xcos²y-cos²xsin²y/ //这个是平方差公式a²-b²=(a+b)(a-b)