Octadecagonal Number

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

An Octadecagonal number is a class of figurate numbers. It has 18 – sided polygon called Octadecagon. The N^th 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 N^th Octadecagonal number is given by the formula: 

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