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 Aug 24 accepted Is it true that $\forall b \forall c \forall x ((x^2 + bx + c \neq 0) \rightarrow b^2 - 4c < 0)$? Aug 24 asked Is it true that $\forall b \forall c \forall x ((x^2 + bx + c \neq 0) \rightarrow b^2 - 4c < 0)$? Aug 21 accepted Indicating when $|x + y + z| = |x| + |y| + |z|$ holds Aug 21 comment Indicating when $|x + y + z| = |x| + |y| + |z|$ holds Nice approach! Thank you :-) Aug 21 accepted The set of all finite sequences of members of a countable set is also countable Aug 21 asked Indicating when $|x + y + z| = |x| + |y| + |z|$ holds Aug 20 comment Prove that if $x^n = y^n$ and $n$ is even, then $x = y$ or $x = -y$ It doesn't assume anything. The question is not clear. Spivak probably assumed a positive even number. Aug 20 comment Prove that if $x^n = y^n$ and $n$ is even, then $x = y$ or $x = -y$ @WarrenHill: Spivak doesn't say anything about it. Oh my! Aug 20 accepted An incorrect proof by exhaustion Aug 20 accepted Prove that if $x^n = y^n$ and $n$ is even, then $x = y$ or $x = -y$ Aug 20 comment Prove that if $x^n = y^n$ and $n$ is even, then $x = y$ or $x = -y$ @GEdgar: I've proved by induction that for all $n \in N$, if $0 \leq y < x$ then $y^n < x^n$. I've also proved that if $n$ is even then $(-y)^n = y^n$. I think everything has been established. Aug 20 comment Prove that if $x^n = y^n$ and $n$ is even, then $x = y$ or $x = -y$ @AndrewD.Hwang: Nice observation. Thank you! Aug 20 asked Prove that if $x^n = y^n$ and $n$ is even, then $x = y$ or $x = -y$ Aug 5 accepted If $\forall \mathcal F(\bigcup \mathcal F = A \Rightarrow A \in \mathcal F)$ then A has exactly one element Aug 5 comment If $\forall \mathcal F(\bigcup \mathcal F = A \Rightarrow A \in \mathcal F)$ then A has exactly one element Hey! Kalispera :) Aug 5 asked If $\forall \mathcal F(\bigcup \mathcal F = A \Rightarrow A \in \mathcal F)$ then A has exactly one element Aug 2 comment Some questions about the Jech's book (Generalized De Morgan's law and distributive law ) You are welcome. Bear in mind that you might also use a different set notation: $A \cap (\bigcup S) = \bigcup_{X \in S}(A \cap X)$, $A \setminus \bigcup S = \bigcap_{X \in S}(A \setminus X)$, and $A \setminus \bigcap S = \bigcup_{X \in S}(A \setminus X)$. Aug 1 answered Some questions about the Jech's book (Generalized De Morgan's law and distributive law ) Aug 1 awarded Teacher Jul 31 comment Proof for the symmetric difference? @HagenvonEitzen: Suppose $A \cap B \neq \emptyset$. Then we can choose some $x$ such that $x \in A \cap B$. This means that $x \in A$ and $x \in B$. Since $x \in A$, it follows that $x \in A \cup B$. But then since $A \Delta B = A \cup B$, $x \in A \Delta B$, so $x \in A \cup B$ and $x \notin A \cap B$. This contradicts the fact that $x \in A \cap B$. Therefore $A \cap B = \emptyset$.