Area of a n-sided regular polygon with given Radius

Given a regular polygon of N sides with radius(distance from the center to any vertex) R. The task is to find the area of the polygon.
Examples:

Input : r = 9, N = 6
Output : 210.444

Input : r = 8, N = 7
Output : 232.571

In the figure, we see that the polygon can be divided into N equal triangles.
Looking into one of the triangles, we see that the whole angle at the centre can be divided into = 360/N parts.
So, angle t = 180/N.
Looking into one of the triangles, we see, 

h = rcost
a = rsint

We know, 

area of the triangle = (base * height)/2 
                     = r2sin(t)cos(t)
                     = r2*sin(2t)/2

So, area of the polygon: 

A = n * (area of one triangle) 
  = n*r2*sin(2t)/2 
  = n*r2*sin(360/n)/2

Below is the implementation of the above approach: 

C++

// C++ Program to find the area
// of a regular polygon with given radius

#include <bits/stdc++.h>
using namespace std;

// Function to find the area
// of a regular polygon
float polyarea(float n, float r)
{
    // Side and radius cannot be negative
    if (r < 0 && n < 0)
        return -1;

    // Area
    // degree converted to radians
    float A = ((r * r * n) * sin((360 / n) * 3.14159 / 180)) / 2;

    return A;
}

// Driver code
int main()
{
    float r = 9, n = 6;

    cout << polyarea(n, r) << endl;

    return 0;
}

Java

// Java Program to find the area
// of a regular polygon with given radius

import java.util.*;
class GFG
{
    // Function to find the area
    // of a regular polygon
    static double polyarea(double n, double r)
    {
        // Side and radius cannot be negative
        if (r < 0 && n < 0)
            return -1;

        // Area
        // degree converted to radians
        double A = ((r * r * n) * Math.sin((360 / n) * 3.14159 / 180)) / 2;

        return A;
    }

    // Driver code
    public static void main(String []args)
    {
        float r = 9, n = 6;

        System.out.println(polyarea(n, r));


    }
}

// This code is contributed
// By ihritik (Hritik Raj)

Python3

# Python3 Program to find the area
# of a regular polygon with given radius

# form math lib import sin function
from math import sin

# Function to find the area
# of a regular polygon
def polyarea(n, r) :

    # Side and radius cannot be negative
    if (r < 0 and n < 0) :
        return -1

    # Area
    # degree converted to radians
    A = (((r * r * n) * sin((360 / n) *
                 3.14159 / 180)) / 2);

    return round(A, 3)

# Driver code
if __name__ == "__main__" :

    r, n = 9, 6
    print(polyarea(n, r))

# This code is contributed by Ryuga

C#

// C# Program to find the area
// of a regular polygon with given radius

using System;
class GFG
{
    // Function to find the area
    // of a regular polygon
    static double polyarea(double n, double r)
    {
        // Side and radius cannot be negative
        if (r < 0 && n < 0)
            return -1;

        // Area
        // degree converted to radians
        double A = ((r * r * n) * Math.Sin((360 / n) * 3.14159 / 180)) / 2;

        return A;
    }

    // Driver code
    public static void Main()
    {
        float r = 9, n = 6;

        Console.WriteLine(polyarea(n, r));


    }
}

// This code is contributed
// By ihritik (Hritik Raj)

PHP

<?php
// PHP Program to find the area of a
// regular polygon with given radius

// Function to find the area
// of a regular polygon
function polyarea($n, $r)
{
    // Side and radius cannot be negative
    if ($r < 0 && $n < 0)
        return -1;

    // Area
    // degree converted to radians
    $A = (($r * $r * $n) * sin((360 / $n) *
                     3.14159 / 180)) / 2;

    return $A;
}

// Driver code
$r = 9;
$n = 6;
echo polyarea($n, $r)."\n";

// This code is contributed by ita_c
?>

Javascript

<script>
// javascript Program to find the area
// of a regular polygon with given radius

// Function to find the area
// of a regular polygon
function polyarea(n , r)
{
    // Side and radius cannot be negative
    if (r < 0 && n < 0)
        return -1;

    // Area
    // degree converted to radians
    var A = ((r * r * n) * Math.sin((360 / n) * 3.14159 / 180)) / 2;

    return A;
}

// Driver code
var r = 9, n = 6;

document.write(polyarea(n, r).toFixed(5));


// This code contributed by Princi Singh
</script>

Output:

210.444

Time Complexity: O(1)

Auxiliary Space: O(1)