题解 | #等差数列#
等差数列
http://www.nowcoder.com/practice/f792cb014ed0474fb8f53389e7d9c07f
数列求和公式
import java.util.Scanner;
public class Main{
public static void main(String[] args){
Scanner in = new Scanner(System.in);
while(in.hasNextInt()){
int a = in.nextInt();
int sum =(3*a+1)*a/2;
System.out.println(sum);
}
}
} 