The smallest element in A is equal to 2a, and the largest element in B is less than 1. If there is no * * * same element between them, then it must be.
2a≥ 1 = >A≥ 1/2 in the same way as a+3 ≤ 5 = > a≤2
The value range of ∴ a is 1/2≤a≤2 or a ∈ [1/2,2].