题解 | #等差数列#
等差数列
http://www.nowcoder.com/practice/f792cb014ed0474fb8f53389e7d9c07f
首先要知道等差数列的公式
an = a1 + (n - 1) * d
sn = n * (a1 + an) / 2
let n = parseInt(readline())
let a1 = 2, d = 3
let an = a1 + (n - 1) * d
let sn = n * (a1 + an) / 2
console.log(sn)
