# Hendecagonal number

Given a number n, the task is to find the nth Hendecagonal number.

A Hendecagonal number is a figurate number that extends the concept of triangular and square numbers to the decagon (Eleven -sided polygon). The nth hendecagonal number counts the number of dots in a pattern of n nested decagons, all sharing a common corner, where the ith hendecagon in the pattern has sides made of i dots spaced one unit apart from each other.

**Examples:**

Input : 2

Output :11

Input :6

Output :141

Formula for nth hendecagonal number :

## C++

`// C++ program to find nth ` `// Hendecagonal number ` `#include <bits/stdc++.h> ` `using` `namespace` `std; ` ` ` `// Function to find ` `// Hendecagonal number ` `int` `hendecagonal_num(` `int` `n) ` `{ ` ` ` `// Formula to calculate nth ` ` ` `// Hendecagonal number ` ` ` `return` `(9 * n * n - 7 * n) / 2; ` `} ` ` ` `// Driver Code ` `int` `main() ` `{ ` ` ` `int` `n = 3; ` ` ` `cout << n << ` `"rd Hendecagonal number: "` `; ` ` ` `cout << hendecagonal_num(n); ` ` ` `cout << endl; ` ` ` `n = 10; ` ` ` `cout << n << ` `"th Hendecagonal number: "` `; ` ` ` `cout << hendecagonal_num(n); ` ` ` ` ` `return` `0; ` `} ` |

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## Java

`// Java program to find nth ` `// Hendecagonal number ` `import` `java.io.*; ` ` ` `class` `GFG ` `{ ` ` ` `// Function to find ` `// Hendecagonal number ` `static` `int` `hendecagonal_num(` `int` `n) ` `{ ` ` ` `// Formula to calculate nth ` ` ` `// Hendecagonal number ` ` ` `return` `(` `9` `* n * n - ` ` ` `7` `* n) / ` `2` `; ` `} ` ` ` `// Driver Code ` `public` `static` `void` `main (String[] args) ` `{ ` `int` `n = ` `3` `; ` `System.out.print(n + ` `"rd Hendecagonal "` `+ ` ` ` `"number: "` `); ` `System.out.println(hendecagonal_num(n)); ` ` ` `n = ` `10` `; ` `System.out.print(n + ` `"th Hendecagonal "` `+ ` ` ` `"number: "` `); ` `System.out.println(hendecagonal_num(n)); ` `} ` `} ` ` ` `// This code is contributed by ajit ` |

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## Python3

`# Program to find nth ` `# Hendecagonal number ` ` ` `# Function of Hendecagonal ` `# number ` `def` `hendecagonal_num(n) : ` ` ` ` ` `# Formula to calculate nth ` ` ` `# Hendecagonal number & ` ` ` `# return it into main function. ` ` ` ` ` `return` `(` `9` `*` `n ` `*` `n ` `-` ` ` `7` `*` `n) ` `/` `/` `2` ` ` `# Driver Code ` `if` `__name__ ` `=` `=` `'__main__'` `: ` ` ` ` ` `n ` `=` `3` ` ` `print` `(n,` `"rd Hendecagonal number : "` `, ` ` ` `hendecagonal_num(n)) ` ` ` ` ` `n ` `=` `10` ` ` `print` `(n,` `"th Hendecagonal number : "` `, ` ` ` `hendecagonal_num(n)) ` ` ` `# This code is contributed by ajit ` |

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## C#

`// C# program to find nth ` `// Hendecagonal number ` `using` `System; ` ` ` `class` `GFG ` `{ ` `// Function to find ` `// Hendecagonal number ` `static` `int` `hendecagonal_num(` `int` `n) ` `{ ` ` ` `// Formula to calculate nth ` ` ` `// Hendecagonal number ` ` ` `return` `(9 * n * n - 7 * n) / 2; ` `} ` ` ` `// Driver Code ` `static` `public` `void` `Main () ` `{ ` ` ` `int` `n = 3; ` ` ` `Console.Write(n + ` ` ` `"rd Hendecagonal number: "` `); ` ` ` `Console.WriteLine( hendecagonal_num(n)); ` ` ` ` ` `n = 10; ` ` ` `Console.Write(n + ` ` ` `"th Hendecagonal number: "` `); ` ` ` `Console.WriteLine( hendecagonal_num(n)); ` ` ` `} ` `} ` ` ` `// This code is contributed by aj_36 ` |

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## PHP

`<?php ` `// PHP program to find nth ` `// Hendecagonal number ` ` ` `// Function to find ` `// Hendecagonal number ` ` ` `function` `hendecagonal_num(` `$n` `) ` `{ ` ` ` ` ` `// Formula to calculate nth ` ` ` `// Hendecagonal number ` ` ` `return` `(9 * ` `$n` `* ` `$n` `- 7 * ` `$n` `) / 2; ` `} ` ` ` `// Driver Code ` `$n` `= 3; ` `echo` `$n` `, ` `"th Hendecagonal number: "` `; ` `echo` `hendecagonal_num(` `$n` `); ` `echo` `"\n"` `; ` ` ` `$n` `= 10; ` `echo` `$n` `, ` `"th Hendecagonal number: "` `; ` `echo` `hendecagonal_num(` `$n` `); ` ` ` `// This code is contributed by m_kit ` `?> ` |

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**Output :**

3th Hendecagonal number: 30 10th Hendecagonal number: 415

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

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