> I'm not a big fan of that picture. We're > trying to compute f'(400), > Well, technically speaking, you are not supposed to talk about derivatives in the pic

> I'm not a big fan of that picture. We're > trying to compute f'(400), > Well, technically speaking, you are not supposed to talk about derivatives in the picture answer. I am just trying to connect the notion of "download rate" with the "slope of the file size function". The triangle with the hypotenuse touching the function (tangent line) is a way to compute the slope from the graph. As such the actual size of the triangle is not important -- so long as it helps you compute the rise-over-run. So to answer your question -- the triangle is not meant to illustrate the derivative calculation. Indeed, it would be quite difficult to show an infinitely small triangle ;)