I was answering this multiple choice question from this website examtimequiz.com/maths-mcq-on-sets:
If $A$ is any set, then
- $A \cup A' = U$
- None of these
- $A \cap A' = U$
- $A \cup A' = \emptyset$
I answered (1), but apparently the correct answer is (3). Why?
And is the empty (null) set finite or infinite?