Theorem 3.1: Let f be an everywhere function from a to b and a and a be subsets of a. then &(1)IfA,cA2, then f(ACf(A,2) (2)f(A1A2)f(A1)f(A2) (3)f(A1UA2)=f(A1)∪f(A2) (4)f(A1)-f(A2)∈f(A1-A2) o Proof:(3)(afADUf(AcA, UA2) 令(b)fA1UA2)∈fA1Uf(42)❖Theorem 3.1: Let f be an everywhere function from A to B, and A1 and A2 be subsets of A. Then ❖(1)If A1A2 , then f(A1 ) f(A2 ) ❖(2) f(A1∩A2 ) f(A1 )∩f(A2 ) ❖(3) f(A1∪A2 )= f(A1 )∪f(A2 ) ❖(4) f(A1 )- f(A2 ) f(A1 -A2 ) ❖ Proof: (3)(a) f(A1 )∪f (A2 ) f(A1∪A2 ) ❖ (b) f(A1∪A2 ) f(A1 )∪f (A2 )