sinx=1/2 siny=1/3 ,sin(x+y)(sinx-y)=?
问题描述：
sinx=1/2 siny=1/3 ,sin(x+y)(sinx-y)=?
数学达人帮我啊·······
答
sin(x+y)sin(x-y)=[sinxcosy+cosxsiny][sinxcosy-cosxsiny]
=(sinxcosy)^2-(cosxsiny)^2
=(sinx)^2[1-(siny)^2]-[1-(sinx)^2](siny)^2
=1/4*(1-1/9)-(1-1/4)*1/9
=5/36