In fact, there is one line, called the tangent line, that touches the curve at exactly one point. The slope of
the tangent line is equal to the slope of the curve at exactly this point. The object of using the above
formula, therefore, is to shrink h down to an infinitesimally small amount. If we could do that, then the
difference between (x 1 + h) and x 1 would be a point.
Graphically, it looks like the following: