sinx+siny=4/5,平方
(sinx)^2+2sinxsiny+(siny)^2=16/25.1
cosx+cosy=3/5 平方
(cosx)^2+2cosxcosy+(cosy)^2=9/25.2
1式+2式得
2+2cosxcosy+2sinxsiny=1
2cosxcosy+2sinxsiny=-1
cosxcosy+sinxsiny=-1/2
cos(x-y)=-1/2