Choosing Vertices of a Polygon from Random Points on a Circle: How Many Ways?

[physicsss]

"Number of ways" question

A circle has random points on its circumferance. How many ways can you form a quadrilateral, a triangle, and a octagon using these points?

I have no idea how to get an numerical answers for this question without using n to represent the number of points on the circle.

Thanks.

 

[GregA]

I think using 'n' is fine...just make a correct formula such that for any n you can get a specific answer.

 

[arunbg]

Of course you need n.
In how many ways can you choose the vertices of an r sided polygon from n points ?