geometry
posted by please help .
distint points a, b, c, d, e, and f are marked on a circle. How many triangles can be formed using these points as vertices?

Draw a circle, form as many triangles as you can, and count.

ok i tried that that its hard why don't u try and tellme the answer ?

You have 6 points on a circle.
It is not possible to have 3 of them forming a straight line (not a triangle) so the question becomes how many choices are there of 3 elements form 6
or C(6,3) = 6!/(3!3!) = 20