Instantaneous Rate of Change
Type in the function of your choice inside the green box. You may enter in special characters by clicking on the grey "a" box within. Next, enter a value for x_1 for which you want to know the instantaneous rate of change. Enter a second value x_2 to draw a secant line. Then use the red slider to drag the secant line closer and closer to the tangent line. When x_2 equals x_1 the Instantaneous Rate of Change (Slope of the Tangent Line) will appear!
What happens as x_2 gets really really close to x_1?