The following applet can be used to illustrate how the chain rule applies to the derivative of f(g(x)) at x=a. Simply enter the functions g(x) and f(x) and the value a. The applet automatically draws the graphs of g(x), f(x) and f(g(x)). The appropriate line tangent to the graph is drawn in red. For g(x), the tangent line goes through the point (a,g(a)), for f(x), the tangent line goes through the point (g(a),f(g(a))) and for f(g(x)), the tangent line goes through the point (a,f(g(a))). The value of a can be changed by simply typing a new value, such as "1.2345", "pi/2", "sqrt(5)+cos(3)", etc. You may also change this value by using the up/down arrow keys or dragging the corresponding point left or right. To move the center of the graph, simply drag any point to a new location. To label the x-axis in radians (i.e. multiples of pi), click on the graph and press "control-r". To switch back, simply press "control-r" again.