[lang-en]To see the previous post[/lang-en][lang-ko]이전 글 보러가기[/lang-ko] [lang-en]By axiom of extensionality, and are the same sets. But commonly, we often consider an ordered pair, and we need the concept of ordered pair to describe more things. Hence, we should define ordered pair, but how can we do that? First of all, let's think about properties that ordered pairs ..