Theorem 3.9: Let f be one-to-one correspondence between A and B 8 Then 冷(1)f1)=f 冷(2f1户=IA 冷(3)f°f1=IB 冷Pro0f:(1)f1)l=f 冷(2f1=IA 令Letf:A→ B and g:B→A, %Is g the inverse function off? 令∫g?= IB andg of?=IA❖ Theorem 3.9: Let f be one-to-one correspondence between A and B. ❖ Then ❖ (1)(f -1 ) -1= f ❖ (2)f -1  f=IA ❖ (3)f  f -1=IB ❖ Proof: (1)(f -1 ) -1= f ❖ (2)f -1  f=IA ❖ Let f:A→B and g:B→A， ❖ Is g the inverse function of f ? ❖ f g?=IB and g  f ?=IA