# Length of Diagonal of a n-sided regular polygon

Given a n-sided regular polygon of side length **a**.The task is to find the length of it’s diagonal.**Examples:**

Input:a = 9, n = 10Output:17.119Input:a = 4, n = 5Output:6.47213

**Approach**:

We know that the sum of interior angles of a polygon =

(n â€“ 2) * 180where,nis the no. of sides in the polygon.

So, each interior angle =(n â€“ 2) * 180/n

Now, we have to findBC = 2 * x. If we draw a perpendicularAOonBC, we will see that the perpendicular bisectsBCinBOandOC, as trianglesAOBandAOCare congruent to each other.

Now,t = (n â€“ 2) * 180/2n

So,sint = x/a

Therefore,x = asint

Hence, diagonal=2x = 2asint = 2asin((n â€“ 2) * 180/2n)

## C++

`// C++ Program to find the diagonal` `// of a regular polygon with given side length` `#include <bits/stdc++.h>` `using` `namespace` `std;` `// Function to find the diagonal` `// of a regular polygon` `float` `polydiagonal(` `float` `n, ` `float` `a)` `{` ` ` `// Side and side length cannot be negative` ` ` `if` `(a < 0 && n < 0)` ` ` `return` `-1;` ` ` `// diagonal` ` ` `// degree converted to radians` ` ` `return` `2 * a * ` `sin` `((((n - 2) * 180) / (2 * n)) * 3.14159 / 180);` `}` `// Driver code` `int` `main()` `{` ` ` `float` `a = 9, n = 10;` ` ` `cout << polydiagonal(n, a) << endl;` ` ` `return` `0;` `}` |

## Java

`// Java Program to find the diagonal` `// of a regular polygon with given side length` `class` `GFG {` `// Function to find the diagonal` `// of a regular polygon` ` ` `static` `float` `polydiagonal(` `float` `n, ` `float` `a) {` ` ` `// Side and side length cannot be negative` ` ` `if` `(a < ` `0` `&& n < ` `0` `) {` ` ` `return` `-` `1` `;` ` ` `}` ` ` `// diagonal` ` ` `// degree converted to radians` ` ` `return` `(` `float` `) (` `2` `* a * Math.sin((((n - ` `2` `) * ` `180` `) / (` `2` `* n)) * ` `3.14159` `/ ` `180` `));` ` ` `}` `// Driver code` ` ` `public` `static` `void` `main(String[] args) {` ` ` `float` `a = ` `9` `, n = ` `10` `;` ` ` `System.out.printf(` `"%.3f"` `,polydiagonal(n, a));` ` ` `}` `}` `// This code is contributed by 29AjayKumar` |

## Python3

`# Python3 Program to find the diagonal` `# of a regular polygon with given side length` `import` `math as mt` `# Function to find the diagonal` `# of a regular polygon` `def` `polydiagonal(n, a):` ` ` `# Side and side length cannot` ` ` `# be negative` ` ` `if` `(a < ` `0` `and` `n < ` `0` `):` ` ` `return` `-` `1` ` ` `# diagonal degree converted to radians` ` ` `return` `(` `2` `*` `a ` `*` `mt.sin((((n ` `-` `2` `) ` `*` `180` `) ` `/` ` ` `(` `2` `*` `n)) ` `*` `3.14159` `/` `180` `))` `# Driver code` `a, n ` `=` `9` `, ` `10` `print` `(polydiagonal(n, a))` `# This code is contributed` `# by Mohit kumar 29` |

## C#

`// C# Program to find the diagonal` `// of a regular polygon with given side length` `using` `System;` `public` `class` `GFG{` ` ` `// Function to find the diagonal` `// of a regular polygon` ` ` `static` `float` `polydiagonal(` `float` `n, ` `float` `a) {` ` ` `// Side and side length cannot be negative` ` ` `if` `(a < 0 && n < 0) {` ` ` `return` `-1;` ` ` `}` ` ` `// diagonal` ` ` `// degree converted to radians` ` ` `return` `(` `float` `) (2 * a * Math.Sin((((n - 2) * 180) / (2 * n)) * 3.14159 / 180));` ` ` `}` `// Driver code` ` ` `static` `public` `void` `Main (){` ` ` `float` `a = 9, n = 10;` ` ` `Console.WriteLine(polydiagonal(n, a));` ` ` `}` `}` `// This code is contributed by @Sachin...` |

## PHP

`<?php` `// PHP Program to find the diagonal of a` `// regular polygon with given side length` `// Function to find the diagonal` `// of a regular polygon` `function` `polydiagonal (` `$n` `, ` `$a` `)` `{` ` ` `// Side and side length cannot` ` ` `// be negative` ` ` `if` `(` `$a` `< 0 && ` `$n` `< 0)` ` ` `return` `-1;` ` ` `// diagonal` ` ` `// degree converted to radians` ` ` `return` `2 * ` `$a` `* sin((((` `$n` `- 2) * 180) /` ` ` `(2 * ` `$n` `)) * 3.14159 / 180);` `}` `// Driver code` `$a` `= 9;` `$n` `= 10;` `echo` `polydiagonal(` `$n` `, ` `$a` `);` `// This code is contributed` `// by Sach_Code` `?>` |

## Javascript

`<script>` `// javascript Program to find the diagonal` `// of a regular polygon with given side length` `// Function to find the diagonal` `// of a regular polygon` `function` `polydiagonal(n , a) {` ` ` `// Side and side length cannot be negative` ` ` `if` `(a < 0 && n < 0) {` ` ` `return` `-1;` ` ` `}` ` ` `// diagonal` ` ` `// degree converted to radians` ` ` `return` `(2 * a * Math.sin((((n - 2) * 180)` ` ` `/ (2 * n)) * 3.14159 / 180));` `}` `// Driver code` ` ` `var` `a = 9, n = 10;` `document.write(polydiagonal(n, a).toFixed(3));` `// This code contributed by Princi Singh` `</script>` |

**Output:**

17.119