There are 2n equal points on the circumference (n is greater than 2), and a triangle can be obtained by taking any three points.

The number of right triangles is

C(n，2)*C(4，3)= n *(n- 1)/2 * 4 = 2n *(n- 1)

Or, in another way:

C(n， 1)*C(2n-2， 1)= n *(2n-2)= 2n *(n- 1)