In my textbook, the proof for demorgan's law,
(AintersectionB)* = A*unionB*
[*=complement]
starts with,saying that for all x belongs to (AintersectionB)* , x is not a member of AunionB.
But how can we say that, for example,
if A = {1,2,3} and B = (2,3,4,5} and U = {1,2,3,4,5}...