Let S = {n1*a1, n2*a2, ..., nk*ak}, and let r be a natural number such that there is at

least one r-combination of S. Show that in applying the inclusion-exclusion principle to determine the number of r-combinations of S, one has A1 n A2 n ... n Ak = EmptySet.

n = intersection

Thanks in advance for any help on this one...