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# Icosihenagonal Number

• Last Updated : 17 Mar, 2021

Given a number N, the task is to find Nth Icosihenagonal number.

An Icosihenagonal number is class of figurate number. It has 21 – sided polygon called Icosihenagon. The n-th Icosihenagonal number counts the 21 number of dots and all others dots are surrounding with a common sharing corner and make a pattern. The first few Icosihenagonal numbers are 1, 21, 60, 118, 195, 291, 406 …

Examples:

Input: N = 2
Output: 21
Explanation:
The second Icosihenagonal number is 21
Input: N = 6
Output: 291 Approach: In mathematics, the Nth Icosihenagonal number is given by the formula:

` `

Below is the implementation of the above approach:

## C++

 `// C++ program to find nth``// Icosihenagonal number` `#include ``using` `namespace` `std;` `// Function to find``// Icosihenagonal number``int` `Icosihenagonal_num(``int` `n)``{``    ``// Formula to calculate nth``    ``// Icosihenagonal number``    ``return` `(19 * n * n - 17 * n) / 2;``}` `// Driver Code``int` `main()``{``    ``int` `n = 3;``    ``cout << Icosihenagonal_num(n) << endl;` `    ``n = 10;``    ``cout << Icosihenagonal_num(n) << endl;` `    ``return` `0;``}`

## Java

 `// Java program to find nth``// Icosihenagonal number``class` `GFG{` `// Function to find``// Icosihenagonal number``static` `int` `Icosihenagonal_num(``int` `n)``{``    ``// Formula to calculate nth``    ``// Icosihenagonal number``    ``return` `(``19` `* n * n - ``17` `* n) / ``2``;``}` `// Driver Code``public` `static` `void` `main(String[] args)``{``    ``int` `n = ``3``;``    ``System.out.print(Icosihenagonal_num(n) + ``"\n"``);` `    ``n = ``10``;``    ``System.out.print(Icosihenagonal_num(n) + ``"\n"``);``}``}` `// This code is contributed by Rajput-Ji`

## Python3

 `# Python3 program to find nth``# icosihenagonal number` `# Function to find``# icosihenagonal number``def` `Icosihenagonal_num(n):``    ` `    ``# Formula to calculate nth``    ``# icosihenagonal number``    ``return` `(``19` `*` `n ``*` `n ``-` `17` `*` `n) ``/` `2``    ` `# Driver Code``n ``=` `3``print``(``int``(Icosihenagonal_num(n)))` `n ``=` `10``print``(``int``(Icosihenagonal_num(n)))` `# This code is contributed by divyeshrabadiya07`

## C#

 `// C# program to find nth``// Icosihenagonal number``using` `System;` `class` `GFG{` `// Function to find``// Icosihenagonal number``static` `int` `Icosihenagonal_num(``int` `n)``{``    ``// Formula to calculate nth``    ``// Icosihenagonal number``    ``return` `(19 * n * n - 17 * n) / 2;``}` `// Driver Code``public` `static` `void` `Main()``{``    ``int` `n = 3;``    ``Console.Write(Icosihenagonal_num(n) + ``"\n"``);` `    ``n = 10;``    ``Console.Write(Icosihenagonal_num(n) + ``"\n"``);``}``}` `// This code is contributed by Code_Mech`

## Javascript

 ``
Output:
```60
865```

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