sina-sinβ=1/3(1)
cosa-cosβ=1/2 (2)
(1)平方+(2)平方
sin²a-2sinasinb+sin²b+cos²a-2cosacosb+cos²b=1/9+1/4=13/36
sinasinb+cosacosb=59/72
cos(a-b)=59/72