,l 4G 1 18%018:02 พ. 11 ธ.ค.เสร็จสิ้นScanned Documents.pdf99CHAPTER 2. FUNCTIONSTheorem 2.2.5 Let f: A - B be a function. IfYC Y' C Bthen f-1(Y) C f-I(Y').Proof: We must show that every element of f-'(Y) is in f-'(Y').Let z e f-I(Y). By definition of preimage f(r) E Y, and sinceYCY', f(2)EY'. Then z e f-'(Y') by definition of preimage.Hence f-i(Y) c f-'(Y'), which completes the proof.Look at what we did in the above proof. We started with anelement r e f-(Y) and wed we evaluated what that means to us. Thedefnition of the preimage must be used. It tells us that z mapsinto Y. That is, f(zThe subset hypothesis is now usedf(a) EY C Y' implies that f(r) Er)EY'. Since t maps into Y' thenar is in the preimage, the set of all that maps into Y': ae f-(Y).Hence f-'(Y) C f-'(Y').Notice the detail with which I am arguing. This detail is nec-essary for two reasons. First, we cannot know what the other isthinking so I must show all of the thoughts that I want you tothink. Second, before we can skip steps without fear of error set-ting in, we must first pay our dues with this type of detail. I hopeyou are paying dues. The later material will more than make up forthe present effort I am asking you to exert.Theorem 2.2.6 Let f: A-, B be a function and let X C A.Then X C f-I(f(X)).Picture (2.8) is an illustration of what we will show in the proofof this theorem.Proof: We must show that each element of X is an element off-'(f(X)). Leta e X. By the definition of image f(1) E f(X).For the sake of clarity let W = f(X). Then f(a) eW. Thedefinition of preimage and the definition of W show us thatzEf~(W)=f~'(f(X)).Thus X c f-'(f(X)), which completes the proof.Once again we will detail and motivate the given proof.Toprove that X C f~I(f(X)) we must show that each given z E X