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# Number of triangles formed by joining vertices of n-sided polygon with one side common

• Difficulty Level : Easy
• Last Updated : 07 Apr, 2021

Given N-sided polygon we need to find the number of triangles formed by joining the vertices of the given polygon with exactly one side being common.
Examples:

Input :
Output : 12
The image below is of a triangle forming inside a Hexagon by joining vertices as shown above. The two triangles formed has one side (AB) common with that of a polygon.It depicts that with one edge of a hexagon we can make two triangles with one side common. We can’t take C or F as our third vertex as it will make 2 sides common with the hexagon. Triangle with one side common of a hexagon

No of triangles formed ie equal to 12 as there are 6 edges in a hexagon.
Input :
Output :

Approach :

• To make a triangle with one side common with a polygon the two vertices adjacent to the chosen common vertices cannot be considered as the third vertex of a triangle.
• First select any one edge from the polygon. Consider this edge to be the common edge. Number of ways to select an edge in a polygon would be equal to n.
• Now ,to form a triangle ,select any of the (n-4) vertices left .Two vertices of the common edge and two vertices adjacent to the common edge cannot be considered.
• Number of triangle formed by joining the vertices of an n-sided polygon with one side common would be equal to n * ( n – 4) .

Below is the implementation of the above approach:

## C++

 `// C++ program to implement``// the above problem``#include ``using` `namespace` `std;` `// Function to find the number of triangles``void` `findTriangles(``int` `n)``{``    ``int` `num;``    ``num = n * (n - 4);` `    ``// print the number of triangles``    ``cout << num;``}` `// Driver code``int` `main()``{``    ``// initialize the number of sides of a polygon``    ``int` `n;``    ``n = 6;` `    ``findTriangles(n);` `    ``return` `0;``}`

## Java

 `// Java program to implement``// the above problem``class` `GFG``{``    ` `// Function to find the number of triangles``static` `void` `findTriangles(``int` `n)``{``    ``int` `num;``    ``num = n * (n - ``4``);` `    ``// print the number of triangles``    ``System.out.println(num);``}` `// Driver code``public` `static` `void` `main(String [] args)``{``    ``// initialize the number of sides of a polygon``    ``int` `n;``    ``n = ``6``;` `    ``findTriangles(n);` `}``}` `// This code is contributed by ihritik`

## Python3

 `# Python3 program to implement``# the above problem` `# Function to find the number of triangles``def` `findTriangles(n):``    ``num ``=` `n ``*` `(n ``-` `4``)` `    ``# print the number of triangles``    ``print``(num)` `# Driver code``# initialize the number of sides of a polygon``n ``=` `6` `findTriangles(n)` `# This codee is contributed by mohit kumar 29`

## C#

 `// C# program to implement``// the above problem``using` `System;` `class` `GFG``{``    ` `// Function to find the number of triangles``static` `void` `findTriangles(``int` `n)``{``    ``int` `num;``    ``num = n * (n - 4);` `    ``// print the number of triangles``    ``Console.WriteLine(num);``}` `// Driver code``public` `static` `void` `Main()``{``    ``// initialize the number of sides of a polygon``    ``int` `n;``    ``n = 6;` `    ``findTriangles(n);` `}``}` `// This code is contributed by ihritik`

## Javascript

 ``
Output:
`12`

Time Complexity: O(1)

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