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