How many triangles can be drawn by joining any three vertices of a pen...
Required number of traingles =

Here, n = number of items and
k = number you are picking at time
∴ n = 5, k = 3

How many triangles can be drawn by joining any three vertices of a pen...
Number of Triangles in a Pentagon
There are a total of 5 vertices in a pentagon. To form a triangle, we need to select any 3 vertices out of these 5 vertices.
Calculating the Number of Triangles
- If we select 3 vertices that are not adjacent to each other, we can form 1 triangle.
- If we select 2 adjacent vertices and 1 non-adjacent vertex, we can form 2 triangles.
- If we select 2 non-adjacent vertices and 1 adjacent vertex, we can form 2 triangles.
- If we select 3 adjacent vertices, we can form 1 triangle.
Therefore, the total number of triangles that can be formed by joining any three vertices of a pentagon is 1 + 2 + 2 + 1 = 6.
Additional Triangles
However, we need to consider the triangles formed by the diagonals of the pentagon as well. Each diagonal forms a triangle with one side of the pentagon.
- There are 5 diagonals in a pentagon.
- Each diagonal forms 1 triangle with one side of the pentagon.
- Therefore, the number of additional triangles formed by the diagonals is 5.
Total Number of Triangles
Adding the triangles formed by selecting vertices and the triangles formed by the diagonals, we get a total of 6 + 5 = 11 triangles that can be drawn by joining any three vertices of a pentagon.
Therefore, the correct answer is option D) 11.