The picture is exactly the same—only the notation has changed.
The Slope of a Curve
Suppose that instead of finding the slope of a line, we wanted to find the slope of a curve. Here, the slope
formula no longer works because the distance from one point to the other is along a curve, not a straight
line. But we could find the approximate slope if we took the slope of the line between the two points.
This is called the secant line.