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[SOLVED] Set Theory


Active member
Jan 8, 2013
Dear Everybody,

I am struggling now for determining if the following statements are true or false. If the statement is true, then prove it. If not, make a counterexample.
Here are the statements:
  1. $A \subset B$ and $A \subset C$ if and only if $A \subset (B\cup C)$.
  2. $A \subset B$ or $A \subset C$ if and only if $A \subset (B\cap C)$.

My attemption:
Let A={1,2,3}, B={1,2,4}, and C={3}.
  1. I believe this is true. $A\subset B$ and $A\subset C$. Thus $A \subset (B\cup C)$ is true.
  2. I don't know how to begin.


Well-known member
Apr 22, 2018
Hi Cbarker1 .

The two statements are in fact false, thus you just need to find a counterexample to disprove each. Hints:

  1. The $\Rightarrow$ part is true but not the $\Leftarrow$ part. Consider $A\subset B$, $A\ne\emptyset$, and $C=\emptyset$.
  2. This time $\Leftarrow$ is true but not $\Rightarrow$. Consider $A\subset B$, $A\ne\emptyset$, and $B\cap C=\emptyset$.