2.补集与全集的性质
(1)∁UU=______;
(2)∁U ∅=______;
(3)∁U (∁U A)=______;(4)A∪∁U A=______; (5)A∩∁U A=______. 练习 2 :已知全集 U ={1,2,3,4,5,6,7} ,A ={2,4,5} ,B = {1,3,6,7},则 A∩∁U B=________,(∁U A)∩(∁U B)=________.
∅
{2,4,5}