09CHAPTER 2. FUNCTIONSConversely, we will prove the reverse inclusionr(Ur)cบrmCnENnEN1rer~(Uy)nENThe definition of preimage implies thatf(az) aUx2.3nENand the defnition of U implies that f(r) E Y for some k E N.FunctiThen z E f-'(Y:) for that k E N, and soz2 deftakesrEUf(Y).nENConsequently,long lifront onumbein line,r(Ux) c บr"m)VnENwe seeand thereforeOne p40herd anENr(us) - Urmmgoathea functThis completes the proof.the payour mental mathematical muscles.Here are a few exercises that will buildhas stoThe reader can sample the thrillcreative and discovery processes bygoats.providing proofs of all these results. Rather than repeat the hypotheses overhe hasand over again, let us agree that, for each of these exercises, f: A-- B is aare assfunction, that X, X' C A, and that Y,Y' C B.stones1. If Xc X'C A then f(X) C f(X').as mar2. f(f-I(Y)) CY.보3. If X,X'C A then f(Xn x) c f(X)nf(X).