qqp question1: How can it be proven that [math]x+y=x\times y=\frac{x}{y}[/math]? question2: Set Theory: How do we prove, rigorously, that if [math]X \subseteq Y[/math], then [math]X \cup Y=Y[/math] — and the same for the case [math]X \cap Y=X[/math]?