In general, the product of two polynomials with degrees m and n, respectively, is given by
where we use the convention that ai = 0 for all integers i > m and bj = 0 for all integers j > n. By the binomial theorem,
Using the binomial theorem also for the exponents m and n, and then the above formula for the product of polynomials, we obtain
where the above convention for the coefficients of the polynomials agrees with the definition of the binomial coefficients, because both give zero for all i > m and j > n, respectively.
By comparing coefficients of xr, Vandermonde's identity follows for all integers r with 0 ≤ r ≤ m + n. For larger integers r, both sides of Vandermonde's identity are zero due to the definition of binomial coefficients.
Vandermonde's identity also admits a combinatorial double counting proof, as follows. Suppose a committee consists of m men and n women. In how many ways can a subcommittee of r members be formed? The answer is
The answer is also the sum over all possible values of k, of the number of subcommittees consisting of k men and r − k women:
Take a rectangular grid of r x (m+n-r) squares. There are
paths that start on the bottom left vertex and, moving only upwards or rightwards, end at the top right vertex (this is because r right moves and m+n-r up moves must be made (or vice-versa) in any order, and the total path length is m+n). Call the bottom left vertex (0,0).
There are paths starting at (0,0) that end at (k,m), as k right moves and m-k upward moves must be made (and the path length is m). Similarly, there are paths starting at (k,m) that end at (r,m+n-r), as a total of r right moves must be made and the path length must be m+n. Thus there are
paths that start at (0,0), end at (r,m+n-r), and go through (k,m). This is a subset of all paths that start at (0,0) and end at (r,m+n-r), so sum from k=0 to k=r (as the point (k,m) is confined to be within the square) to obtain the total number of paths that start at (0,0) and end at (r,m+n-r).
Generalized Vandermonde's identity
If in the algebraic derivation above more than two polynomials are used, it results in the generalized Vandermonde's identity. For y + 1 polynomials:
The hypergeometric probability distribution
When both sides have been divided by the expression on the left, so that the sum is 1, then the terms of the sum may be interpreted as probabilities. The resulting probability distribution is the hypergeometric distribution. That is the probability distribution of the number of red marbles in r draws without replacement from an urn containing n red and m blue marbles.
The identity generalizes to non-integer arguments. In this case, it is known as the Chu–Vandermonde identity (see Askey 1975, pp. 59–60) and takes the form
in which form it is clearly recognizable as an umbral variant of the binomial theorem. (For more on umbral variants of the binomial theorem, see binomial type) The Chu–Vandermonde identity can also be seen to be a special case of Gauss's hypergeometric theorem, which states that
The Rothe–Hagen identity is a further generalization of this identity.