求出1 + 11 + 111 + 1111 +…..至n项的和。

在这里，我们将找到级数1 + 11 + 111 + 1111 +…..upto N个项的总和(其中N是给定的)。
例子 ：

Input : 3
Output : 1 + 11 + 111 +....
Total sum is : 123

Input : 4
Output : 1 + 11 + 111 + 1111 +..... 
Total sum is : 1234

Input : 7
Output : 1 + 11 + 111 + 1111 + 11111 + 
         111111 + 1111111 +..... 
Total sum is : 1234567

在这里，我们看到，当N的值为3时，序列的最后值为1 + 11 + 111，即三个项，其和为123。
查找上述系列的总和的程序：

C++

// C++ program to find the sum of
// the series 1+11+111+1111+....
#include 
using namespace std;
 
// Function for finding summation
int summation(int n)
{
    int sum = 0, j = 1;
    for (int i = 1; i <= n; i++) {
        sum = sum + j;
 
        // Appending a 1 at the end
        j = (j * 10) + 1;
    }
 
    return sum;
}
 
// Driver Code
int main()
{
    int n = 5;
    cout << " " <<  summation(n);
    return 0;
}
 
// This code is contributed by shivanisinghss2110

C

// C program to find the sum of
// the series 1+11+111+1111+....
#include 
 
// Function for finding summation
int summation(int n)
{
    int sum = 0, j = 1;
    for (int i = 1; i <= n; i++) {
        sum = sum + j;
 
        // Appending a 1 at the end
        j = (j * 10) + 1;
    }
 
    return sum;
}
 
// Driver Code
int main()
{
    int n = 5;
    printf("%d", summation(n));
    return 0;
}

Java

// Java program to find the sum of
// the series 1+11+111+1111+....
import java.io.*;
 
class GFG
{
 
    // Function for finding summation
    static int summation(int n)
    {
        int sum = 0, j = 1;
        for (int i = 1; i <= n; i++)
        {
            sum = sum + j;
            j = (j * 10) + 1;
        }
 
        return sum;
    }
 
    // Driver Code
    public static void main(String args[])
    {
        int n = 5;
        System.out.println(summation(n));
    }
}
 
// This code is contributed
// by Nikita Tiwari

Python

# Python program to get the summation
# of following series
def summation(n):
    sum = 0
    j = 1
     
    for i in range(1, n + 1):
        sum = sum + j
        j = (j * 10) + 1
         
    return sum
         
# Driver Code
n = 5
print(summation(n))

C#

// C# program to find the sum of
// the series 1+11+111+1111+....
using System;
 
class GFG
{
 
    // Function for finding summation
    static int summation(int n)
    {
        int sum = 0, j = 1;
        for (int i = 1; i <= n; i++)
        {
            sum = sum + j;
            j = (j * 10) + 1;
        }
 
        return sum;
    }
 
    // Driver Code
    public static void Main()
    {
        int n = 5;
        Console.WriteLine(summation(n));
    }
}
 
// This code is contributed by vt_m

Javascript

 
// Javascript program to find the sum of
// the series 1+11+111+1111+....
 
    // Function for finding summation
    function summation( n) {
        let sum = 0, j = 1;
        for ( let i = 1; i <= n; i++) {
            sum = sum + j;
             
            // Appending a 1 at the end
            j = (j * 10) + 1;
        }
 
        return sum;
    }
 
    // Driver Code
      
        let n = 5;
        document.write(summation(n));
         
// This code contributed by Princi Singh
 

输出 ：

12345

另一种方法：让给定级数S = 1 + 11 + 111 + 1111 +。 。 。 +至第n个学期。用公式求级数之和。

$\\ S = 1 + 11 + 111 + 1111 + . . . + upto \ n \ term \\ \\ S = \left ( \frac{1}{9} \right )*\left ( 9 + 99 + 999 + 9999 + . . . + upto \ n \ term \right ) \\ \\ S = \left ( \frac{1}{9} \right ) * \left ( \left (10 ^{1} - 1\right ) + \left (10 ^{2} -1 \right ) +\left (10 ^{3} -1 \right ) + \left (10 ^{4} -1 \right ) + . . . + \left (10 ^{n} -1 \right ) \right ) \\ \\ S = \left ( \frac{1}{9} \right ) *\left (10 ^{1} + 10 ^{2} + 10 ^{3} + 10 ^{4} + . . . + 10 ^{n} - n\right ) \\ \\ S = \left ( \frac{1}{9} \right ) * \left ( \frac{10 * (10^{n} - 1)}{10 - 1} - n \right ) \\ \\ S = \frac{10^{n+1} - 10 - 9n}{81}$
 

下面是上述方法的实现。

C++

// C++ program to find the sum of
// the series 1+11+111+1111+....
#include 
 
// Function for finding summation
int summation(int n)
{
    int sum;
 
    sum = (pow(10, n + 1) -
               10 - (9 * n)) / 81;
 
    return sum;
}
 
// Driver Code
int main()
{
    int n = 5;
    printf("%d", summation(n));
    return 0;
}

Java

// java program to find the sum of
// the series 1+11+111+1111+....
import java.io.*;
 
class GFG {
 
    // Function for finding summation
    static int summation(int n)
    {
        int sum;
     
        sum = (int)(Math.pow(10, n + 1) -
                10 - (9 * n)) / 81;
     
        return sum;
    }
     
    // Driver Code
    public static void main (String[] args)
    {
        int n = 5;
        System.out.println(summation(n));
    }
}
 
// This code is contributed by anuj_67.

Python3

# Python3 program to
# find the sum of
# the series 1+11+111+1111+....
import math
 
# Function for
# finding summation
def summation(n):
    return int((pow(10, n + 1) -
                    10 - (9 * n)) / 81);
 
# Driver Code
print(summation(5));
 
# This code is contributed
# by mits.

C＃

// C# program to find the sum of
// the series 1+11+111+1111+....
using System;
 
class GFG {
 
    // Function for finding summation
    static int summation(int n)
    {
        int sum;
     
        sum = (int)(Math.Pow(10, n + 1) -
                10 - (9 * n)) / 81;
     
        return sum;
    }
     
    // Driver Code
    public static void Main ()
    {
        int n = 5;
        Console.WriteLine(summation(n));
    }
}
 
// This code is contributed by anuj_67.

Java脚本

// javascript program to find the sum of
// the series 1+11+111+1111+....
 
// Function for finding summation
function summation( n)
{
    let sum;
    sum = (Math.pow(10, n + 1) -
               10 - (9 * n)) / 81;
    return sum;
}
 
// Driver Code
let n = 5;
    document.write(summation(n)) ;
     
// This code is contributed by aashish1995
 

输出 ：

12345