シグマの計算公式について

$${\displaystyle \sum_{k=1}^{n}c=cn}$$

$${\displaystyle \sum_{k=1}^{n}k=\dfrac{1}{2}n(n+1)}$$

$${\displaystyle \sum_{k=1}^{n}k^{2}=\dfrac{1}{6}n(n+1)(2n+1)}$$

$${\displaystyle \sum_{k=1}^{n}k^{3}=\left\{\dfrac{1}{2}n(n+1)\right\}^{2}}$$