# 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  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; }

## C

 // C program to find nth // Hendecagonal number #include    // 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;     printf("%drd Hendecagonal number: ",n);     printf("%d\n",hendecagonal_num(n));       n = 10;     printf("%dth Hendecagonal number: ",n);     printf("%d\n",hendecagonal_num(n));       return 0; }   // This code is contributed by kothavvsaakash.

## 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

## 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

## 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

## PHP

 

## Javascript

 

Output :

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

Time Complexity: O(1)
Auxiliary Space: O(1)
Reference: https://en.wikipedia.org/wiki/Polygonal_number

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