MHB Proof of Sets: Proving (i) and (ii)

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The discussion focuses on proving two statements about power sets. It is concluded that statement (i), which claims that if Z = X ∪ Y, then P(Z) = P(X) ∪ P(Y), is false, as demonstrated by a counterexample. Conversely, statement (ii), asserting that if Z = X ∩ Y, then P(Z) = P(X) ∩ P(Y), is true, supported by logical reasoning about subset relationships. The participants agree on the validity of these conclusions based on their examples and proofs. Understanding these properties of power sets is essential in set theory.
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If $X$ is a set, then the power set $P(X)$ of a set is the set of all subsets of $X$.

I need to decide whether the following statements are true or false and prove it:
(i) If $Z = X \cup Y$ , then $P(Z) = P(X) \cup P(Y)$.

(ii) If $Z = X \cap Y$ , then $P(Z) = P(X) \cap P(Y)$.

By examples I think that (ii) is true but (i) is false. However, I have no idea how to prove it.

Note: Example which I used was: $X=\left\{a,b\right\}$ and $Y=\left\{b,c\right\}$.

(i) $Z = X \cup Y =\left\{a,b,c\right\}$ , so $P(Z) = \left\{\left\{\right\},\left\{a\right\},\left\{b\right\},\left\{c\right\},\left\{a,b\right\},\left\{a,c\right\},\left\{b,c\right\},\left\{a,b,c\right\}\right\}$ .

However, $P(X) \cup P(Y) = \left\{\left\{\right\},\left\{a\right\},\left\{b\right\},\left\{c\right\},\left\{a,b\right\},\left\{b,c\right\}\right\}$.

That's why I disagree with a statement (i).

(ii) $Z = X \cap Y =\left\{b\right\}$ , so $P(Z) = \left\{\left\{\right\},\left\{b\right\}\right\}$ . However, $P(X) \cap P(Y) = \left\{\left\{\right\},\left\{b\right\}\right\}$ as well.

That's why I think a statement (ii) is true.

I would be grateful for help!
 
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Mathick said:
That's why I disagree with a statement (i).
You are absolutely correct.

If $Z=X\cap Y$, then $A\subseteq Z\iff A\subseteq X\land A\subseteq Y$ for any $A$, which means that $A\in P(Z)\iff A\in P(X)\land A\in P(Y)\iff A\in P(X)\cap P(Y)$.
 
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