题解 | #等差数列#

了解等差数列公式即可：

等差数列{an}的通项公式为：an=a1+(n-1)d

前n项和公式为：Sn=n*a1+n(n-1)d/2或Sn=n(a1+an)/2

import java.util.*;

public class Main{
    public static void main(String args[]){
        Scanner sc = new Scanner(System.in);

        int n = sc.nextInt();
        add(n);
    }

    public static void add(int n){
        int sum = 0;
        int a;
        for (int i = 0; i < n; i++) {
            a = 2 + (n - 1) * 3;
            sum = n * (2 + a) / 2;
        }

        System.out.println(sum);
    }
}