For master\'s theorem T(n) = a*T(n/b) + f(n) I am using 3 cases:
a*f(n/b) = c*f(n) for some constant c > 1 then
You might find these three cases from the Wikipedia article on the Master theorem a bit more useful:
Now there is no direct dependence on the choice of n anymore - all that matters is the long-term growth rate of f and how it relates to the constants a and b. Without seeing more specifics of the particular recurrence you're trying to solve, I can't offer any more specific advice.
Hope this helps!