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The purpose of this lab is to use Maple to help you visualize and
understand the **mean value theorem** (MVT) and its applications.

The **mean value theorem** is one of the most important and useful
theorems from calculus. Before stating it, here is a (rather silly)
application of it.

Just after you get on the Pike by Auburn, you pass a car pulled over by the highway patrol. Feeling glad that it isn't you, and relatively safe because you have a radar detector, you blast on by. One hour and eighty miles down the road, however, another patrolman pulls you over. You're not worried, because your detector had warned you to slow down. Imagine your shock, however, when he proceeds to write you a ticket for going 80 miles per hour! When you protest, he tells you that the patrolman at Auburn had radioed your position ahead. He then claims that since your average speed was eighty, the MVT says you must have been going eighty at least once in your journey. Then you see the ``MathNet'' patch on his uniform, and you really start to get that sinking feeling.

Mathematically, the MVT can be stated as follows.

**Theorem 308** **(MVT) **

Suppose that *f*(*x*) is continuous on the closed
interval [*a*,*b*] and differentiable on the open interval (*a*,*b*). Then
for some *c* between *a* and *b*,

Geometrically, the MVT says that for at least one point strictly
between *a* and *b*, the slope of the tangent line to the graph of
*f*(*x*) is equal to the slope of the straight line between the two
points (*a*,*f*(*a*)) and (*b*,*f*(*b*)). (Note: This line is often called the
*secant line* between *a* and *b*.) Obtaining a formula for the secant
line isn't hard, but to cut down on the amount of Maple drudgery, a
procedure, called `secantline` has been written that does this for you.
The `secantline` command takes three arguments. The first is a
function or expression, the next one is the base point, and the third
is the increment, *b*-*a*.
Before you can use this procedure, you must first load it using the
`with(CalcP)` command, as shown in the following example session.

> with(CalcP):

> f := x -> x^3;

> g := secantline(f(x),x=-1,2);

> plot({f(x),g},x=-1..1);

Looking at the graph displayed in the previous example, you should be
able to see that there are two values of *x* between *x*=-1 and *x*=1
where the slope of *f*(*x*) = *x ^{3}* is the same as the slope of the secant
line between the two points (-1,

To find these values, we need to solve for the values of *x* in the
interval (-1,1) where the derivative, *f*'(*x*), is equal to the slope
of the secant line, which is 1 in this example. Maple
commands to do this, and plot the two tangent lines are shown
below. In this case the `solve` command finds both solutions. Note
the use of the label `sol` and the notation `sol[1]` and `
sol[2]` to access the two roots.

> sol := solve(diff(f(x),x)=1,x);

> plot({f(x),g,tangentline(f(x),x=sol[1]), tangentline(f(x),x=sol[2])},x=-1..1);

The `solve` command can't always do the job if the function *f*(*x*)
is complicated. When the `solve` command fails, there is no output
from the command, as shown in the example below. In this case, use the
`fsolve` command instead, as shown below.

> f := x -> sin(x^2);

> g := secantline(f(x),x=4,1);

> plot({f(x),g},x=4..5);

> solve(diff(f(x),x)=diff(g,x),x);

> fsolve(diff(f(x),x)=diff(g,x),x=4..5);

It is important to restrict the *x* interval in the `
fsolve` command to (4,5) in this case, because the equation

If you do this for the previous example, , you
should be able to recognize that one of the roots is in the interval
(4,4.3). Using this interval in the `fsolve` command produces
the following results.

> fsolve(diff(f(x),x)=diff(g,x),x=4..4.3);

There is nothing special about the interval (4,4.3); any other
interval that included the root at , but didn't
include any other roots would have worked. That is, the interval
(4.1,4.2) would have worked equally as well, but the interval
(4,4.6) is *not* a good choice because it includes two roots.

- 1.
- For each of the following functions, verify graphically that the
MVT holds and find all values of
*c*,*a*<*c*<*b*, that satisfy the condition- (a)
*f*(*x*) =*x*+^{3}*x*/2-2^{2}*x*+1,*a*= -2,*b*= 3.- (b)
- ,
*a*=-1,*b*=2. - (c)
- ,
*a*=0,*b*=2.

- 2.
- Explain how the MVT applies to support the patrolman's claim.
- 3.
- In some cases, the slope of the secant line can be interpreted
as an average value. For example consider the falling body problem
from the text. If
*s*(*t*) is the position of a body, then the quantity is change in position over the change in time, or the average velocity of the body over the time interval. Because the derivative of*s*(*t*) is the velocity*v*(*t*), the mean value theorem says that there is at least one time between*a*and*b*at which the instantaneous velocity is equal to the average velocity.Suppose that a body starts at a height of 150 feet and that air resistance can be neglected. At time zero, the body is released and subsequently falls to the ground.

- (a)
- Find the time at which the body hits the ground.
- (b)
- Find the average velocity of the body from when it was released to when it hits the ground.
- (c)
- Find the time at which the instantaneous velocity is equal to the average velocity.

- 4.
- In the previous problem, you should have found that the time at
which the instantaneous velocity is equal to the average velocity is
exactly half the time it takes for the body to hit the ground. In this
problem, we add an initial upward velocity to the body and see if this
is still true. To help you along, we have broken this up into
- (a)
- Suppose that the body starts at the same position of 150 feet
above the ground, but has an intial upward velocity of 20 ft/sec. This
means that the position function is
*s*(*t*) = -16*t*+ 20^{2}*t*+ 150. Find the time at which the body hits the ground. - (b)
- Using the value of
*t*you just computed, find the average velocity of the body. - (c)
- Use
*v*(*t*) = -32*t*+20 to compute the value of*t*at which the instantaneous velocity is equal to the average velocity. Compare this value to the time it takes for the body to hit the ground.

3/16/2000