A line l is an asymptote to a curve if the distance from a point P to the line l tends to zero as P tends to infinity along some unbounded part of the curve. Consider the following examples:
Example (i) has x =−2 and x =1 as vertical asymptotes and y = 0 as a horizontal asymptote. Example (ii) has no vertical asymptotes and y = 3 as a horizontal asymptote. To investigate example (iii), it can be rewritten as
Then it can be seen that y = x−1 is a slant (or oblique) asymptote, that is, an asymptote that is neither vertical nor horizontal.