cos(a+b)=1/5,cos(a
c(a+b)=1/5,cos(a-b)=3/5. 求tanatanb的值
cos(a+b)=cosacosb-sinasinb=1/5, cos(a-b)=cosacosb+sinasinb=3/5, 所以cosacosb=2/5,sinasinb=1/5, 所以tanatanb=1/2.