已知cos(a+b)=1/3,cos(a-b)=1/2,则了log(底数是5)(tana*tanb)=?
问题描述：
已知cos(a+b)=1/3,cos(a-b)=1/2,则了log(底数是5)(tana*tanb)=?
答
cos(a+b)=cosacosb-sinasinb=1/3
cos(a-b)=cosacosb+sinasinb=1/2
联立--->cosacosb=5/12,sinasinb=1/12--->tanatanb=1/5
--->log5_(tanatanb) = -1