sot 集合 The set a is said to be a proper subset of B ifAcB and there is a∈ B and agA A是B的真子集、 B真包含A、 A真包含在B中 2/24/202111:15PM Deren Chen Zhejiang univS e t s 集合 2/24/2021 11:15 PM Deren Chen, Zhejiang Univ. 10 The set A is said to be a proper subset of B if AB and there is aB and aA A是B的真子集、 B真包含A、 A真包含在B中