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We have a slightly different way of expressing the binomial theorem in combinatorics. Let n be a positive integer. Then for all x and y,

(x + y)^n = sigma (n) x^k y^n-k,
                               (k)

where the sum is taken from k=0 to k=n, and (n) is is a binomial coefficient.
                                                                                 (k)

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