If f .x/ is the position of a particle at time x ¤ x0, the difference quotientf .x/ f .x0/x x0is the average velocity of the particle between times x0 and x. As x approaches x0, theaverage applies to shorter and shorter intervals. Therefore, itmakes sense to regard the limit(2.3.1), if it exists, as the particle’s instantaneous velocity at time x0. This interpretationmay be useful even if x is not time, so we often regard f 0.x0/ as the instantaneous rate ofchange of f .x/ at x0, regardless of the specific nature of the variable x. The derivative alsohas a geometric interpretation. The equation of the line through two points .x0; f .x0// and.x1; f .x1// on the curve y D f .x/ (Figure 2.3.1) is