cos(a+b)=1/3.(1)
cos(a-b)=1/5 .(2)
coscosb-sinasinb = 1/3 .(3)
coscosb+sinasinb = 1/5 .(4)
{(4）-（3）}÷2 得：sinasinb = -1/15 .(5)
{(4）+（3）}÷2 得：cosacosb = 4/15 .(6)
(5)÷(6)得：tana*tanb = -1/4
答案错了!