等差数列の和
https://gyazo.com/4be0404f9acb46b35ddd9817300091cc
$ \sum_{a=1}^{A}a = 1/2*A(A+1)
$ n(a+l)/2 項数:$ n, 初項:$ a, 末項:$ l, 公差:$ d
等差数列