If `A xx B = phi`, then is it necessary that both A and B are empty?

A-B= phi iff

A cistern can be filled up in 4 h by an inlet A. An outlet B can empty the cistern in 8 h. If both A and B are opened simultaneously, then after how much time will the cistern get filled?

If A-B=phi and B-C=B, then A nn C

If A and B are two sets such that A xx B = phi , then which of the following must be true ?

If A - B =phi phi and B-A = phi then what can we conclude ?

If A nnn B'=phi then prove that A=A nn B and hence show that A sube B .

If A nn C=phi, then prove that (A xx B)nn(C xx B)=phi