Math 130-03, Lab 6

Exercise 1

Sometimes it's easy to forget that the derivative of a function is a function in its own right. It has a graph. You can find its value at different inputs. It might have a formula. Just as you can ask where the function is increasing and decreasing, you can ask where the derivative is increasing and decreasing.

The following applet shows both the graph of a function and the graph of its derivative. It also gives a formula for the derivative, and it shows a tangent line on the graph of $ y=f(x)$ as well as the corresponding point on the graph of $ y=f'(x)$. You can change the position of the tangent line and point by clicking-and-dragging on either graph.

Use this applet to look at the derivatives of several functions, including: $ x^2$, $ x^3$, $ 2*x$, $ x/3+1$, $ sqrt(x)$, $ tan(x)$, $ x^x$, and $ 2^x+cos(x)$. Answer the following questions:

(a) What happens on the graph of $ y=f'(x)$ when the tangent line to $ y=f(x)$ is horizontal? Why?

(b) What happens to the tangent line to the graph of $ y=f(x)$ when the point $ (x,f'(x))$ lies above the x-axis? Why?

(c) What is the derivative of a linear function such as $ f(x)=m*x+b$? Explain why this is true, based on the fact that the derivative of a function gives the slope of that function at any given point.

(d) If you drag the mouse from left-to-right on the graph, the tangent line seems to rotate, either clockwise or counterclockwise. Can you relate the direction of rotation to some property of the graph of $ f$, or of the graph of $ f'$, or both? If you can find the connection, try to explain why it holds.

Exercise 2

This question continues the discussion of taxes that was begun in the previous lab. Here is the same table that you looked at last week: The instructions for Form 1040 (Federal Income Tax) include the following table for computing the amount of tax, based on your taxable income. (This is the table for use by a single taxpayer).

If income is over But not over Then tax is of amount over
$0 $26,250 15% $0
26,250 63,550 $3,937.50 + 28% 26,250
63,550 132,600 $14,381.50 + 31% 63,550
132,600 288,350 $35,787.00 + 36% 132,600
288,350   $91,857.00 + 39.6% 288,350

(a) This table defines a function, $ T$, where $ T(x)$ is the amount of tax paid on a taxable income of $ x$ dollars. What is the derivative, $ T'(x)$, of this function? Write your answer as a split function. (Note that on each of the intervals in its definition, $ T$ is a linear function.)

(b) In Economics, $ T'(x)$ is called the marginal tax rate for income $ x$. Sometimes, economists define the marginal tax rate for income $ x$ to be $ T(x+1)-T(x)$. That is, it is the amount of tax that you would pay on the next dollar of income. Explain why these two definitions are essentially the same. That is, explain why $ T'(x)$ and $ T(x+1)-T(x)$ are at least approximately equal. (Keep in mind that one dollar is generally a very small amount, compared to the total amount of income, $ x$.)

(c) For any ``fair'' tax system it is certainly true that the marginal tax rate must be less than or equal to 1. Explain why this is true.

(d) What other properties do you think the marginal tax rate should have in a ``fair'' tax system Should $ T'(x)$ always be greater than zero? Should $ T'(x)$ get larger as $ x$ gets larger? Explain.

Exercise 3

For this exercise, you will work with an applet that you've seen before. It shows the graphs of two functions, $ f$ and $ g$, together with the graph of their composition, $ g\circ f$. This time, however, the applet also shows some tangent lines. To open the applet in a separate, resizable window, click this button:

To use the applet, you can drag the red square on the graph of $ f$. This sets the input value, $ x$. The applet shows the tangent lines to the three curves at the points $ (x,f(x))$, $ (f(x),g(f(x))$, and $ (x,g(f(x))$. Try moving the red square to see how everything changes. Try some other functions. Try turning on ``Use Mouse'' for one or both functions, if you like. (In this mode, you can define the functions by dragging points on the graph up and down.) When you feel that you know what is going on, do the following exercises.

(a) Set up the applet so that both $ f$ and $ g$ are linear functions, such as $ f(x)=3*x+1$ and $ g(x)=2*x-3$. Observe the slopes of the tangent lines. Do this for three or more pairs of linear functions. How does the slope of the composition $ g(f(x))$ compare to the slopes of the functions $ f(x)$ and $ g(x)$?

(b) Suppose that $ f(x)=a*x+b$ and $ g(x)=c*x+d$, where $ a$, $ b$, $ c$, and $ d$ are constants. Calculate a formula for $ g(f(x))$. Based on that formula, what is the slope of the graph of $ g(f(x))$? Does the answer agree with your observation from part (a)?

(c) Now, turn from lines to more general functions. For a line, the derivative is a constant and the value of the derivative is the same as the slope of the line. For most functions, however, the derivative varies from one point to another. However, you can still compare the slopes of the tangent lines for any given input value.

Look at several pairs of functions. For each pair of functions, look at several different x-values by dragging the red square to several different positions. In each case, record the slopes of the tangent lines to $ f$, $ g$, and $ g\circ f$. Check whether you get the same sort of pattern that you saw in part (a). (For example, you can try this for $ f(x)=sin(x)$ and $ g(x)=x^2$, for $ f(x)=g(x)=x^2$, and for $ f(x)=x*trunc(x)$ and $ g(x)=x^2$.)

(d) Suppose that $ h(x)=g(f(x))$. Based on your observations, what would be the formula for $ h'(x)$, in terms of the derivatives of $ f$ and $ g$. (Remember that to get a slope out of a derivative, you have to plug in an input value. The input value in $ h'(x)$ is $ x$. Be careful to say which input values are going into the derivatives of $ f$ and $ g$ in your formulas.)

David Eck, March 2001