The expansion of the two sides of a sum equality in terms of Polynomials in and , followed by closed form summation in terms of and . For an example of the technique, see Bloom (1995).
References
Bloom, D. M. ``A Semi-Unfriendly Identity.'' Problem 10206. Solution by R. J. Chapman. Amer. Math. Monthly 102, 657-658, 1995.
Wilf, H. S. Generatingfunctionology, 2nd ed. New York: Academic Press, 1993.