Mathematics for IGCSE & O level - Sets (Section 1 - No. 6)
What does it mean to say set A is a subset of set B (A ⊆ B)?
All elements in A are also in B.
All elements in B are also in A.
A and B have no elements in common.
A can be equal to B.
Giải thích
A subset means all elements in the set are in another set, which could mean the sets are equal.