题解 | #等差数列#
等差数列
https://www.nowcoder.com/practice/f792cb014ed0474fb8f53389e7d9c07f
等差数列前n项和通项公式:(首项 + 尾项)* 项数 / 2
等差数列通项公式:尾项 = 首项 + (项数 - 1) * 等差
import java.util.Scanner;
public class Main{
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
int n = sc.nextInt();
System.out.print((3 * n + 1) * n / 2);
}
}

