Question 4. (a) Prove algebraically that 3 n 2 - (C) - (A) + (A) + (A) + + ("=) + (") 3) = 2 Hint: you can use that (%) + (6+1) = (a+11). (b) Use a combinatorial argument to show that 2 3 2 n- 2 n - (6) - (6) + (2) + (1) +---+ (") + (") 2 2 Hint: If your set is {1, 2,. ,n} and you choose 3 elements out of it, ... think of the largest element in your subset.