3. The fact that vertical lines have infinite slope is an annoying inconvience. It means that we can't use computations involving slopes for infinite lines. Thus, if a result uses computations involving slopes, then we will have to deal with the case of vertical lines separately. Fortunately, the equation of a vertical line is actually simpler than the general equation, so the inconvenience of having to fashion a separate argument for vertical lines is ameliorated by the fact that the argument for the simpler equations of vertical lines are usually simpler than the general case.