In this app, you will set the tangent line visually to the correct slope at a selected point on the graph. A small blue dot appears at the same [math]x[/math]-value as the tangent point, with a [math]y[/math]-value equal to the slope of your tangent line. Click the "Mark Point" button to freeze that point. Now drag the red tangent point somewhere else on the graph and repeat. Do this for several points along the graph. When you are done, click the "Show f ' " box. The derivative will appear. If you did a good job estimating the tangent line slopes, the derivative should pass through or nearly through your frozen blue points.[br][br]This helps to illustrate that the derivative f ' is just a "slope function" - it gives the slope (IROC) of a function f at any value of [math]x[/math].