序列2、22、222，……的总和

找到以下序列的总和：2，22，222，………至n个项。
例子 ：

Input : 2
Output: 1.9926

Input : 3
Output: 23.9112

一个简单的解决方案是逐项计算项并添加到结果中。
使用以下公式可以有效地解决上述问题：

C++

// CPP program to find sum of series
// 2, 22, 222, ..
#include 
using namespace std;
 
// function which return the
// the sum of series
float sumOfSeries(int n)
{
    return 0.0246 * (pow(10, n) - 1 - (9 * n));
}
 
// driver code
int main()
{
    int n = 3;
    cout << sumOfSeries(n);
    return 0;
}

Java

// JAVA Code for Sum of the
// sequence 2, 22, 222,...
import java.util.*;
 
class GFG {
     
    // function which return the
    // the sum of series
    static double sumOfSeries(int n)
    {
        return 0.0246 * (Math.pow(10, n)
                            - 1 - (9 * n));
    }
     
    /* Driver program */
    public static void main(String[] args)
    {
         int n = 3;
         System.out.println(sumOfSeries(n));
    }
}
 
// This code is contributed by Arnav Kr. Mandal.

Python3

# Python3 code to find
# sum of series
# 2, 22, 222, ..
import math
 
# function which return
# the sum of series
def sumOfSeries( n ):
    return 0.0246 * (math.pow(10, n) - 1 - (9 * n))
     
# driver code
n = 3
print( sumOfSeries(n))
 
# This code is contributed by "Sharad_Bhardwaj".

C#

// C# Code for Sum of the
// sequence 2, 22, 222,...
using System;
 
class GFG {
     
    // Function which return the
    // the sum of series
    static double sumOfSeries(int n)
    {
        return 0.0246 * (Math.Pow(10, n)
                           - 1 - (9 * n));
    }
     
    // Driver Code
    public static void Main()
    {
        int n = 3;
        Console.Write(sumOfSeries(n));
    }
}
 
// This code is contributed by vt_m.

Javascript

 
// JavaScript program for Sum of the
// sequence 2, 22, 222,...
 
    // function which return the
    // the sum of series
    function sumOfSeries(n)
    {
        return 0.0246 * (Math.pow(10, n)
                            - 1 - (9 * n));
    }
 
// Driver code   
          
        let n = 3;
        document.write(sumOfSeries(n));
             

输出 ：

23.9112