It's not difficult! Because: a/b+b/a=6cosC, so a/b+b/a=6*(a*a+b*b-c*c)/2ab, so there is: c*c=2/3*(a*a+b*b) and because: tanc/tana+tanc.

Ps: Special value method can also be used: let a= 1 and b= 1. After all, it is a fill-in-the-blank question! Hmm. How interesting