Question:

What is the number of triangles that can be formed whose vertices are the vertices of an octagon but have only one side common with that of the octagon?

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To calculate the number of triangles with one side common to the octagon, subtract the triangles that do not have a side common with the octagon from the total possible triangles.
Updated On: Mar 25, 2025
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The Correct Option is B

Solution and Explanation

The number of triangles that can be formed with vertices of an octagon is given by choosing 3 vertices from 8. The total number of triangles is: \[ \binom{8}{3} = \frac{8 \times 7 \times 6}{3 \times 2 \times 1} = 56. \] However, the triangles with one side common with the octagon must include two adjacent vertices. There are 8 sides in the octagon, so there are 8 such triangles. The total number of triangles with one side common with the octagon is: \[ 56 - 53 = 3. \]
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