=cos^2 a + cos^2 b - 2cosacosb + sin^2 a + sin^2 b - 2 sinasinb
=2 - 2(cosacosb+sina+sinb)
=2-2cos(a-b)
因为cos(a-b)属于[-1,1]
所以当cos(a-b)=-1时原式取最大值4