Question about intersection/union of a set and its complement 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?

I am assuming that $A'$ = the complement of set $A$, and that $U$ = the universal set. The red part is the set $A$, and $A'$ is everything in the white. The set $U$ is everything, so the red part + the white part. That makes $A \cup  A' = U$. So there was a mistake in the question and your answer is correct.

