Mathematics
Undergraduate

(4)のK3乗の問題で3行目がどうしてこうなるのか分かりません…

等差数列の和 分からない

Answers

∑k^3のときは
(1/2n(n+1))^2=1/4n^2(n+1)^2

∑3k=3× ∑kのときは
3× 1/2n(n+1)

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