Greetings math gods. This class is killing me. I don't mind when questions involve numbers but I hate stuff like this.

Verify:

Note: the original question was use the factorial definition of C(m,r) to verify that:

C(n,2) = C(k,2) + k(n-k) + C(n-k,2) 1 <= k <= n

I really have no idea where to begin. How do you manipulate this ?