# Number of triangles formed by joining vertices of n-sided polygon with two common sides and no common sides

Given **N**-sided polygon we need to find the total number of triangles formed by joining the vertices of the given polygon with exactly two sides being common and no side being common.

**Examples:**

Input :N = 6

Output :6 2

The image below is of a triangle forming inside a Hexagon by joining vertices as shown above.

The triangle formed has two sides (AB and BC) common with that of a polygon. Similarly BC and

CD can make one triangle. With this, we can say that there will be a total of 6 triangles possible

having two sides common with that of a polygon. The second image of a hexagon,

a triangle is formed with no side common with that of a polygon.

There will be just 2 triangles possible, BFD and ACE.

Number of triangles formed are 6 and 2 with two side common and with no side common respectively.

Input :N = 7

Output :7 7

**Approach :**

- To make a triangle two side common with a polygon we will take any one side of a n-sided polygon, take one vertex of the chosen side and join an edge adjacent to the vertex of the other vertex.
- Traversing through each vertex and adjoining an edge adjacent to the vertex of the other vertex ,there will be
**N**number of triangles having two side common. - Now, to calculate the number of triangles with no side common subtract the total number of triangles with one side common and the total number of triangles with two side from the total number of triangles possible in a polygon.
- Triangles with no common side = Total triangles (
^{n}C_{3}) – one side common triangles ( n * ( n – 4 ) – two side common triangles ( n ). - Thus number of triangles with no common side with the polygon would be equal to n * ( n – 4 ) * ( n – 5 ) / 6.

**Note:**To calculate the number of triangles having one side common with that of a polygon click here

Below is the implementation of the above approach:

## C++

`// C++ program to implement ` `// the above problem ` `#include <bits/stdc++.h> ` `using` `namespace` `std; ` ` ` `// Function to find the number of triangles ` `void` `findTriangles(` `int` `n) ` `{ ` ` ` `int` `num = n; ` ` ` ` ` `// print the number of triangles ` ` ` `// having two side common ` ` ` `cout << num << ` `" "` `; ` ` ` ` ` `// print the number of triangles ` ` ` `// having no side common ` ` ` `cout << num * (num - 4) * (num - 5) / 6; ` `} ` ` ` `// Driver code ` `int` `main() ` `{ ` ` ` `// initialize the number ` ` ` `// of sides of a polygon ` ` ` `int` `n; ` ` ` `n = 6; ` ` ` ` ` `findTriangles(n); ` ` ` ` ` `return` `0; ` `} ` |

*chevron_right*

*filter_none*

## Java

`// Java program to implement ` `// the above problem ` `import` `java.io.*; ` ` ` `class` `GFG ` `{ ` ` ` ` ` `// Function to find the number of triangles ` `static` `void` `findTriangles(` `int` `n) ` `{ ` ` ` `int` `num = n; ` ` ` ` ` `// print the number of triangles ` ` ` `// having two side common ` ` ` `System.out.print( num + ` `" "` `); ` ` ` ` ` `// print the number of triangles ` ` ` `// having no side common ` ` ` `System.out.print( num * (num - ` `4` `) * (num - ` `5` `) / ` `6` `); ` `} ` ` ` `// 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 anuj_67.. ` |

*chevron_right*

*filter_none*

## Python3

`# Python3 program to implement ` `# the above problem ` ` ` `# Function to find the number of triangles ` `def` `findTriangles(n): ` ` ` `num ` `=` `n ` ` ` ` ` ` ` `# print the number of triangles ` ` ` `# having two side common ` ` ` `print` `(num, end ` `=` `" "` `) ` ` ` ` ` `# print the number of triangles ` ` ` `# having no side common ` ` ` `print` `(num ` `*` `(num ` `-` `4` `) ` `*` `(num ` `-` `5` `) ` `/` `/` `6` `) ` ` ` `# Driver code ` ` ` `# initialize the number ` `# of sides of a polygon ` `n ` `=` `6` `; ` ` ` `findTriangles(n) ` ` ` `# This code is contributed by Mohit Kumar ` |

*chevron_right*

*filter_none*

## 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 = n; ` ` ` ` ` `// print the number of triangles ` ` ` `// having two side common ` ` ` `Console.Write( num + ` `" "` `); ` ` ` ` ` `// print the number of triangles ` ` ` `// having no side common ` ` ` `Console.WriteLine( num * (num - 4) * (num - 5) / 6); ` `} ` ` ` `// 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 anuj_67.. ` |

*chevron_right*

*filter_none*

**Output:**

6 2

** Time Complexity: ** O(1)

Don’t stop now and take your learning to the next level. Learn all the important concepts of Data Structures and Algorithms with the help of the most trusted course: **DSA Self Paced**. Become industry ready at a student-friendly price.

## Recommended Posts:

- Number of triangles formed by joining vertices of n-sided polygon with one side common
- Number of ways a convex polygon of n+2 sides can split into triangles by connecting vertices
- Number of cycles formed by joining vertices of n sided polygon at the center
- Count number of triangles possible for the given sides range
- Number of ways to arrange 2*N persons on the two sides of a table with X and Y persons on opposite sides
- Area of the circumcircle of any triangles with sides given
- Check if it is possible to create a polygon with given n sides
- Count of acute, obtuse and right triangles with given sides
- Number of occurrences of a given angle formed using 3 vertices of a n-sided regular polygon
- Minimum side of square embedded in Regular polygon with N sides
- Number of cycles in a Polygon with lines from Centroid to Vertices
- Check if number formed by joining two Numbers is Perfect Cube
- Number of triangles that can be formed with given N points
- Number of triangles formed from a set of points on three lines
- Number of Triangles that can be formed given a set of lines in Euclidean Plane
- Total number of triangles formed when there are H horizontal and V vertical lines
- Find other two sides of a right angle triangle
- Find the type of triangle from the given sides
- Maximize volume of cuboid with given sum of sides
- Check whether triangle is valid or not if sides are given

If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.