Big O Problem

Consider the recurrence

T(1)=a
T(n)=cT(n/d)+bn^k, for n a power of d

Iteratively expand T(n) in terms of T(n/d^i) for i=1,2,.... Show that

a) if c > d^k, then T(n) is O(n^logd c)
b) if c = d^k, then T(n) is O(n^k log n)
c) if c < d^k, then T(n) is O(n^k)