Question

A polygon is convex when no line that contains a side of the polygon contains a point in the interior of the polygon. Consider a convex polygon with n sides.

a. Use the combinations formula to write an expression for the number of diagonals in an n-sided polygon.
b. Use your result from part (a) to write a formula for the number of diagonals of an n -sided convex polygon.

Answer 1

a. To find the number of diagonals in an n-sided polygon, we need to count the number of pairs of vertices that can be connected by a diagonal, while excluding the sides of the polygon. Any pair of vertices that are adjacent or separated by only one vertex are already connected by a side of the polygon, so we need to consider pairs of vertices that are separated by at least two vertices.

We can choose two vertices out of n vertices in the polygon in nC2 ways. However, any pair of vertices that are separated by only one vertex defines a side of the polygon, so we need to subtract n from this count to exclude the n sides of the polygon. Therefore, the number of diagonals in an n-sided polygon is:

nC2 - n = (n * (n-1))/2 - n = (n^2 - 3n)/2

b. The above expression gives the number of diagonals for any polygon, not just a convex one. However, for a convex polygon, any pair of vertices separated by only one vertex defines a side of the polygon, so we need to exclude these diagonals as well.

For an n-sided convex polygon, there are n sides and n vertices, so we have to exclude n diagonals (one diagonal for each vertex). Therefore, the number of diagonals in an n-sided convex polygon is:

(n^2 - 3n)/2 - n = (n^2 - 5n)/2

Explanation: