Division by Zero

Let $$x$$ and $$y$$ be numbers such that $$y = 0$$.

The quantity $$\frac x y$$ is undefined.

It is a common mistake to forget this fact when evaluating formulas.

Note
While $$\frac{0}{0}$$ is undefined, the limit of a rational function whose denominator approaches 0 is not necessarily undefined (see L'Hôpital's Rule).