题解 | #等差数列#

import java.util.Scanner;

public class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int a1 = 2;
        int an=0;
        int n =0;
        int sum = 0;
        while(sc.hasNextInt()){
            n=Integer.valueOf(sc.nextLine());
            an=a1+(n-1)*3;
            sum=(a1+an)*n/2;
            System.out.println(sum);
        }
    }
}

先上代码.求等差数列可以使用(a1+an)n/2的办法,an=a1+(n-1)d,易知公差=3,套用公式求解即可.