// // 累乗の和について \(\displaystyle \sum_{k=1}^n k=\frac{1}{2}n(n+1)\)，\(\displaystyle \sum_{k=1}^n k^2=\frac{1}{6}n(n+1)(2n+1)\)，\(\displaystyle \sum_{k=1}^n k^3=\left \{ \frac{1}{2}n(n+1) \right \}^2\) が成り立ちます．数列\(\{ a_n \…