cos(a+b)=cosacosb-sinasinb
=sinasinb(1/(tanatanb)-1=sinasinb(1/3-1)=(-2/3)sinasinb
=(-1/3)[cos(a-b)-cos(a+b)].1）
cos(a-b)=2cos^2[(a-b)/2]-1 [要用到cos^2x=1/(1+tan^2x)]
=2/{1+tan^2[(a-b)/2)}-1=2/(1+4)-1=-3/5
将cos(a-b)=-3/5代入1）式可得：
-3cos(a+b)=-3/5-cos(a+b)
cos(a+b)=3/10