Master's theorem with f(n)=log n

For master's theorem T(n) = a*T(n/b) + f(n) I am using 3 cases:

1. If a*f(n/b) = c*f(n) for some constant c > 1 then T(n) = (n^log(b) a)
2. If a*f(n/b) = f(n) then T(n) = (f(n) log(b) n)
3. If a*f(n/b) = c*f(n) for some constant c < 1 then T(n) = (f(n))

But when f(n) = log n or n*log n, the value of c is dependent on value of n. How do I solve the recursive function using master's theorem?