can someone help me how to prove this...?
Prove by induction that the number of 2-subsets of an n-set A equals n(n-1)/2. [hint: Let x be any object of A and B=A-{x}. Then a 2-subset of A is either a 2-subset of B or a 1-subset of B. Count the number of subsets in each case.}
-Thanks