a) (n-a)(n-a-1)/2 + (n-a)a

Consider a clique of n-a vertices. Join each vertex of that clique with each of the "a" vertices of the independent set.

b) (n-k+1)(n-k)/2

Consider k-1 isolated vertices, and a clique of n-k+1 vertices.

c) (n-1)(n-2)/2

Consider the case when k=2.