cos(a b)=1/3 cos(a-b)=1/2cos(a b) cos(a-b)=2cosacosb=5/6 cosacosb=5/12cos(a-b)-cos(a b)=2sinasinb=1/6 sinasinb=1/12tanatanb=sinasinb/cosacosb=1/5
