等二十线数

给定一个数字N ，任务是找到^第N^个二十同位数字。

An Icositrigonal number is class of figurate number. It has 23 – sided polygon called Icositrigon. The N-th Icositrigonal number count’s the 23 number of dots and all others dots are surrounding with a common sharing corner and make a pattern. The first few Icositrigonol numbers are 1, 23, 66, 130, 215, 321, 448 … 
 

例子：

Input: N = 2 
Output: 23 
Explanation: 
The second Icositrigonol number is 66. 
Input: N = 6 
Output: 321 
 

方法：第N个同余正数由下式给出：

$Tn = (21n^2 - 19n)/2$

下面是上述方法的实现：

C++

// C++ program to find nth
// Icositrigonal number
 
#include 
using namespace std;
 
// Function to find N-th
// Icositrigonal number
int Icositrigonal_num(int n)
{
    // Formula to calculate nth
    // Icositrigonal number
    return (21 * n * n - 19 * n) / 2;
}
 
// Driver Code
int main()
{
    int n = 3;
    cout << Icositrigonal_num(n) << endl;
     
    n = 10;
    cout << Icositrigonal_num(n);
 
    return 0;
}

Java

// Java program to find nth
// Icositrigonal number
class GFG{
 
// Function to find N-th
// Icositrigonal number   
static int IcositrigonalNum(int n)
{
     
    // Formula to calculate nth
    // Icositrigonal number
    return (21 * n * n - 19 * n) / 2;
}
 
// Driver code
public static void main(String[] args)
{
    int n = 3;
    System.out.print(IcositrigonalNum(n) + "\n");
     
    n = 10;
    System.out.print(IcositrigonalNum(n));
}
}
 
// This code is contributed by spp____

Python3

# Python3 program to find nth
# Icositrigonal number
 
# Function to find N-th
# Icositrigonal number
def IcositrigonalNum(n):
     
    # Formula to calculate nth
    # Icositrigonal number
    return (21 * n * n - 19 * n) / 2;
 
# Driver code
n = 3
print(IcositrigonalNum(n))
 
n = 10
print(IcositrigonalNum(n))
 
# This code is contributed by spp____

C#

// C# program to find nth
// Icositrigonal number
using System;
 
class GFG{
 
// Function to find N-th
// Icositrigonal number    
static int IcositrigonalNum(int n)
{
     
    // Formula to calculate nth
    // Icositrigonal number
    return (21 * n * n - 19 * n) / 2;
}
 
// Driver code
public static void Main()
{
    int n = 3;
    Console.WriteLine(IcositrigonalNum(n));
     
    n = 10;
    Console.WriteLine(IcositrigonalNum(n));
}
}
 
// This code is contributed by spp____

Javascript

 
    // Javascript program to find nth 
    // Icositrigonal number
     
    // Function to find N-th 
    // Icositrigonal number 
    function Icositrigonal_num(n) 
    { 
        // Formula to calculate nth 
        // Icositrigonal number 
        return (21 * n * n - 19 * n) / 2; 
    } 
     
    let n = 3; 
    document.write(Icositrigonal_num(n) + "</br>"); 
       
    n = 10; 
    document.write(Icositrigonal_num(n));
     

输出：

66
955

参考： https : //en.wikipedia.org/wiki/Polygonal_number