## Calculus (3rd Edition)

The slope of the secant line over the interval $[x_0, x_1]$ approaches the slope of the tangent line at $x = x_0$.
The slope of the secant line over the interval $[x_0, x_1]$ approaches the slope of the tangent line at $x = x_0$. Graphically, this means that as we "zoom in" on the curve, it looks more and more like a straight line.