How many triangles can be formed if the vertices are chosen from a set of n points all lying on a circle? I couldn't understand the question.Why the points must lie on the circle?Is the answer C(n,3)?

Since the points all lie on a circle, no three of them can be collinear
with each other.
That means you don't have to worry about whether three points on the same
line can be said to define a "triangle".
Your answer is correct.
