级数之和 0.7, 0.77, 0.777, ... 最多 n 项

给定 n 项数。求系列 0.7, 0.77, 0.777, ... 最多 n 项的总和。
例子：

Input : 2
Output : 1.46286

Input : 3
Output : 2.23609

使用的方法：
让我们用 S 表示系列的总和：

Sum = 0.7 + 0.77 + 0.777 + …. up to n terms
= 7/9(0.9 + 0.99 + 0.999 + … up to n terms)
= 7/9[(1 – 0.1) + (1 – 0.01) + (1-0.001) + … up to n terms]
= 7/9[(1 + 1 + 1… upto n terms) – (1/10 + 1/100 + 1/1000 + … upto n terms)]
= 7/9[n – 0.1 * (1 – (0.1)ⁿ)/(1 – 0.1)]
= 7/81[9n – 1 + 10^-n]

编程需要懂一点英语

以下是查找给定系列总和的实现：

C++

// C++ program for sum of the series 0.7,
// 0.77, 0.777, ... upto n terms
#include 
using namespace std;
 
// function which return the
// the sum of series
float sumOfSeries(int n)
{
    return .086 * (9 * n - 1 +
           pow(10, (-1) * n));
}
 
// Driver code
int main()
{
    int n = 2;
    cout << sumOfSeries(n);
    return 0;
}

Java

// Java program for sum of the series 0.7,
// 0.77, 0.777, ... upto n terms
import java.io.*;
import java.math.*;
 
class GFG {
 
    // function which return the
    // the sum of series
    static float sumOfSeries(int n)
    {
        return .086f * (9 * n - 1 +
        (float)(Math.pow(10, (-1) * n)));
    }
 
    // Driver code
    public static void main(String args[])
    {
        int n = 2;
        System.out.println(sumOfSeries(n));
    }
}
 
/*This code is contributed by Nikita Tiwari.*/

Python3

# Python 3 program for sum of the series 0.7,
# 0.77, 0.777, ... upto n terms
import math
 
# Function which return the
# the sum of series
def sumOfSeries(n) :
    return .086 * (9 * n - 1 + math.pow(10, (-1) * n));
 
 
# Driver code
n = 2
print(sumOfSeries(n))
 
 
# This code is contributed by Nikita Tiwari.

C#

// C# program for sum of the series 
// 0.7, 0.77, 0.777, ... upto n terms
using System;
 
class GFG {
 
    // Function which return the
    // the sum of series
    static float sumOfSeries(int n)
    {
        return .086f * (9 * n - 1 +
               (float)(Math.Pow(10, (-1) * n)));
    }
 
    // Driver code
    public static void Main()
    {
        int n = 2;
        Console.Write(sumOfSeries(n));
    }
}
 
// This code is contributed by vt_m.

PHP

Javascript

输出：

1.46286