Use a combinatorial argument to prove each of the following identities. No marks will be awarded for use of any other method. (a) n(n 1) (-2) = (2)k(k 1) (Hint: Use a committee selection argument where there are special roles.) n (2) 2-k (Hint: Create words using an alphabet of size 3. Count by cases of how many of the first letter in the alphabet appear in the word.) k=0 (b) 3n =