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The binomial theorem expands (a + b)ⁿ as the sum over k of C(n, k)·a^(n−k)·bᵏ. There are n + 1 terms, and the coefficients form row n of Pascal's triangle.
Binomial theorem
(a + b)ⁿ = Σₖ₌₀ⁿ C(n, k)·a^(n−k)·bᵏ
C(5,2)·2³·3² = 10 × 8 × 9 = 720, out of a total of 5⁵ = 3125.
For even n it is k = n/2; for odd n there are two middle terms at k = (n−1)/2 and (n+1)/2.