Generalization of complete bipartite graphs

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The concept of complete bipartite graphs can be generalized to define the complete multipartite graph $K_{r_1, r_2, ..., r_k}$. This graph consists of $k$ sets of vertices $A_1, A_2, ..., A_k$, with $|A_i| = r_i$ for each $i$, where all possible inserted edges are present and no intras-et edges are present. Find expressions for the order and size of $K_{r_1, r_2, ..., r_k}$.

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Pick any one of the sets of vertices. How many edges are there connecting each of it's vertices to the rest of the graph? Now sum over all sets, and realize you are counting each edge twice (looking from each end).