Question: how to tell the difference between a removable discontinuity and a vertical asymptote?
Answer: The difference between a "removable discontinuity" and a "vertical asymptote" is that we have a R. discontinuity if the term that makes the denominator of a rational function equal zero for x = a cancels out under the assumption that x is not equal to a. Othewise, if we can't "cancel" it out, it's a vertical asymptote.