4. [20 pts] Recall that a combinatorial proof for an identity proceeds as follows: 1. State a counting question. 2. Answer the question in two ways: (i) one answer must correspond to the left-hand side (LHS) of the identity (ii) the other answer must correspond to the right-hand side (R.HS). 3. Conclude that the LHS is equal to the RHS. With that in mind, give a combinatorial proof of each the following two identities (a) n -1 = 2(n - 2) + (n -2) 2 where n > 2. (b) itm - 1 mtr m - m where m > 1 and r > 0