四孔数

给定数N ，任务是找到^第N^个四孔菌数。

A Tetracontadigon number is a class of figurate numbers. It has a 42-sided polygon called Tetracontadigon. The N-th Tetracontadigonal number count’s the 42 number of dots and all other dots are surrounding with a common sharing corner and make a pattern. The first few Tetracontadigonol numbers are 1, 42, 123, 244, 405, 606 … 
 

例子：

Input: N = 2 
Output: 42 
Explanation: 
The second Tetracontadigonol number is 42. 
Input: N = 3 
Output: 123 
 

方法：第N个四触角数由下式给出：

• 侧多边形的第N个项= $\frac{((s-2)n^2 - (s-4)n)}{2}$
• 因此42个面的多边形的第N个项是

$Tn =\frac{((42-2)n^2 - (42-4)n)}{2} =\frac{(40n^2 - 38n)}{2}$

下面是上述方法的实现：

C++

// C++ implementation for the
// above approach
#include 
using namespace std;
 
// Function to find the
// nth Tetracontadigonal Number
int TetracontadigonalNum(int n)
{
    return (40 * n * n - 38 * n) / 2;
}
 
// Driver Code
int main()
{
    int n = 3;
    cout << TetracontadigonalNum(n);
 
    return 0;
}

Java

// Java implementation for the
// above approach
import java.util.*;
class GFG{
 
// Function to find the
// nth Tetracontadigonal Number
static int TetracontadigonalNum(int n)
{
    return (40 * n * n - 38 * n) / 2;
}
 
// Driver Code
public static void main(String[] args)
{
    int n = 3;
    System.out.print(TetracontadigonalNum(n));
}
}
 
// This code is contributed by Rajput-Ji

Python3

# Python3 implementation for the
# above approach
 
# Function to find the
# nth tetracontadigonal number
def TetracontadigonalNum(n):
 
    return int((40 * n * n - 38 * n) / 2)
 
# Driver Code
n = 3
print (TetracontadigonalNum(n))
 
# This code is contributed by PratikBasu

C#

// C# implementation for the
// above approach
using System;
class GFG{
 
// Function to find the
// nth Tetracontadigonal Number
static int TetracontadigonalNum(int n)
{
    return (40 * n * n - 38 * n) / 2;
}
 
// Driver Code
public static void Main()
{
    int n = 3;
    Console.Write(TetracontadigonalNum(n));
}
}
 
// This code is contributed by Code_Mech

Javascript

// Javascript implementation for the
// above approach
 
 
    // Function to find the
    // nth Tetracontadigonal Number
    function TetracontadigonalNum( n) {
        return (40 * n * n - 38 * n) / 2;
    }
 
    // Driver Code
      
        let n = 3;
        document.write(TetracontadigonalNum(n));
         
// This code is contributed by todaysgaurav
 

输出：

123

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