I have a certain problem: When you have a function $f(x)$ and it can have some undefined values, the reciprocal function does some things I don't understand. For example: i) $$f(x)=\tan(x) $$ or another one: ii) $$ f(x)=\frac {5}{x-3} $$ Clearly you can see that for ii) $x=3$ will be undefin...