Section 8.2 Definition of the Derivative: What is this graph?

[markbarsamian]

Section 8.2 Definition of the Derivative: What is this graph?

The bottom of the page ends with the following sentence:

"For your viewing pleasure, we have below the graph of y=f(x) and the graph of the tangent line at the point x=20."

There are a number of problems with this.

1. The variable that has been in use in the preceding example about marginal revenue is q, not x

2. The function in the example is called R (for Revenue), not f

3. The graph that you provide shows two lines crossing at x=10. There is no parabola corresponding to the Revenue function, and there is no line tangent at q=20.

I don't see what this graph has to do with the preceding example.