cosa-cosb=1/2
sina-sinb=-1/3
上式两边平方得
(cosa-cosb)^2=1/4
(sina-sinb)^2=(-1/3)^2
展开并相加得
2-2cosacosb-2sinasinb=1/4+1/9=13/36
2-2cos(a-b)=13/36
cos(a-b)=59/72