4) [12 marks,4 marks each] Let f(x) = x3 - 4r. Let S = {re [-5,5] | f(x) 0}. a) Express both S and T as an interval or a union of intervals. b) Find lub(S). What is f(lub(S))? c) Find gib(T). What is f(gib(T))? Note: This exercise relates directly to the key idea in the proof of the Intermediate Value Theorem which we encounter later in the course