(sinx+siny)^2=1/4 ①
(cosx+cosy)^2=1/9 ②
①+ ②：2cosxcosy+(cos x) ²+(cos y) ²+2sinxsiny+(sinx) ²+(siny) ²=13/36
2cosxcosy+2sinxsiny+2=13/36
cosxcosy+sinxsiny=-59/72
即cos(x-y)=-59/72
而cos²((x-y)/2)=[1+cos（x-y）]/2=13/144