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Octadecagonal Number

  • Last Updated : 18 Mar, 2021

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

An Octadecagonal number is a class of figurate numbers. It has 18 – sided polygon called Octadecagon. The Nth Octadecagonal number count’s the eighteen number of dots and all other dots are surrounding with a common sharing corner and make a pattern. The first few Octadecagonal numbers are 1, 18, 51, 100, 165, 246, 343 … 
 

Examples: 
 

Input: N = 2 
Output: 18 
Explanation: 
The second octadecagonal number is 18. 
Input: N = 6 
Output: 246 
 

 



 

Approach: The Nth Octadecagonal number is given by the formula: 
 

Tn = (16n^2 - 14n)/2

Below is the implementation of the above approach:
 

C++




// C++ program to find Nth
// Octadecagonal number
 
#include <bits/stdc++.h>
using namespace std;
 
// Function to find N-th
// Octadecagonal number
int Octadecagonal_num(int n)
{
    // Formula to calculate nth
    // Octadecagonal number
    return (16 * n * n - 14 * n) / 2;
}
 
// Driver code
int main()
{
    int n = 3;
    cout << Octadecagonal_num(n) << endl;
 
    n = 10;
    cout << Octadecagonal_num(n) << endl;
 
    return 0;
}

Java




// Java program to find Nth
// octadecagonal number
import java.io.*;
import java.util.*;
 
class GFG{
     
// Function to find N-th
// octadecagonal number
static int Octadecagonal_num(int n)
{
         
    // Formula to calculate nth
    // octadecagonal number
    return (16 * n * n - 14 * n) / 2;
}
     
// Driver code
public static void main(String[] args)
{
    int n = 3;
    System.out.println(Octadecagonal_num(n));
     
    n = 10;
    System.out.println(Octadecagonal_num(n));
}
}
 
// This code is contributed by coder001

Python3




# Python3 program to find Nth
# octadecagonal number
 
# Function to find N-th
# octadecagonal number
def Octadecagonal_num(n):
     
    # Formula to calculate nth
    # octadecagonal number
    return (16 * n * n - 14 * n) / 2
 
# Driver code   
n = 3
print(int(Octadecagonal_num(n)))
 
n = 10
print(int(Octadecagonal_num(n)))
 
# This code is contributed by divyeshrabadiya07    

C#




// C# program to find Nth
// octadecagonal number
using System;
 
class GFG{
     
// Function to find N-th
// octadecagonal number
static int Octadecagonal_num(int n)
{
         
    // Formula to calculate nth
    // octadecagonal number
    return (16 * n * n - 14 * n) / 2;
}
     
// Driver code
public static void Main(string[] args)
{
    int n = 3;
    Console.Write(Octadecagonal_num(n) + "\n");
     
    n = 10;
    Console.Write(Octadecagonal_num(n) + "\n");
}
}
 
// This code is contributed by rutvik_56

Javascript




<script>
 
    // Javascript program to find Nth
    // Octadecagonal number 
     
    // Function to find N-th
    // Octadecagonal number
    function Octadecagonal_num(n)
    {
     
        // Formula to calculate nth
        // Octadecagonal number
        return (16 * n * n - 14 * n) / 2;
    }
       
    let n = 3;
    document.write(Octadecagonal_num(n) + "</br>");
    n = 10;
    document.write(Octadecagonal_num(n));
     
    // This code is contributed by divyesh072019.
</script>
Output: 
51
730

 

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

 

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