设sin(x+y)=a,sin(x-y)=b,则sinxcosy等于?
问题描述：
设sin(x+y)=a,sin(x-y)=b,则sinxcosy等于?
答
sin(x+y)=a
sinxcosy+sinycosx=a
sin(x-y)=b
sinxcosy-sinycosx=b
两式相加
2sinxcosy=a+b
sinxcosy=（a+b）/2