题解 | #等差数列#
等差数列
http://www.nowcoder.com/practice/f792cb014ed0474fb8f53389e7d9c07f
/*
**确定数列尾项An
**n = (An - A1) / d + 1
**An = (n - 1) * d + A1
*/
while(line = readline()) {
const num = parseInt(line);
console.log(num * (2 + 2 + 3*(num -1)) / 2);
}
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