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Subsections
Suppose that
is a differentiable function. Then we
know that the value of
gives the slope of the tangent
line at
. Geometrically, the slope of the tangent line at a
particular point
tells us whether the value of the function is
increasing, decreasing, or staying the same as we look at
values of
near
. In applications, one is often trying to find
the minimum or maximum values of a function so it turns out to be
important to be able to determine when a function is increasing and
when it is decreasing. Mathematically, we say that a function is
increasing on an interval
if
means
for every pair of numbers
in
. Conversely, we we say that
a function is
decreasing on an interval
if
means
for every pair of numbers
in
. These are the definitions
of increasing and decreasing functions, but they are not very easy to
apply. Most often, we use the first derivative as described in the
following theorem.
Theorem 1
Suppose
is continuous on an interval
and differentiable at
every interior point of
.
- If
on the interior of
, then
is increasing on
.
- If
on the interior of
, then
is decreasing on
.
This theorem says that we can determine when a function is
increasing or decreasing by solving the inequalities
and
. In practice, we usually work with functions having
continuous derivatives, which means that
can change sign only at
a point where
. For example, consider
. The
derivative is
, which is zero only at
. This critical
point divides the real line up into two intervals,
and
. Since
can never be zero if
, the sign of
is constant on each interval. That is for
we have
so
is decreasing for
. Similarly,
is increasing for
. This suggests the following procedure for determining where a
function is increasing or decreasing.
- Find the critical points of
. Note that according to the
definition in the text, critical points of
are points where either
is zero, the derivative doesn't exist, or endpoints of
if
is defined on a finite interval
.
- The critical points divide the domain of
into subintervals
on which the sign of
is constant. Check the sign of
at one
interior point on each subinterval. If it is positive,
is
increasing on that subinterval. If it is negative,
is decreasing
on that subinterval.
The second derivative,
also provides
information about the shape of the curve in terms of what is called
concavity. Concavity can also be defined in several ways. Geometrically,
it can be said that the graph of
is concave up near a point
if the tangent line at
lies below the graph of
on some
open interval containing
and is concave up if the tangent line
lies above the graph of
on some open interval containing
. Algebraically, concavity is most often defined by saying that
is concave up on an interval
if
is increasing on
and is
concave down on
if
is decreasing on
. Using the theorem
above and remembering that
is the derivative of
gives the
following result.
Theorem 2
Suppose
be twice differentiable on the open interval
.
- If
on
, then
is concave up on
.
- If
on
, then
is concave down on
.
This means that we can find where
is concave up and concave down
using the same procedure on
that we used on
. That is, we first find all of values of
for which
, or
doesn't exist. Including the endpoints, if our domain is a
finite interval, these values of
are the endpoints of distinct
subintervals on which the sign of
is constant. Checking the sign
of
at one point in the interior of each subinterval determines
the concavity of
on that subinterval.
Remember also that the second derivative can be helpful in determining local maximums and local minimums. That is, if you find a critical value, where
or is undefined, then substitute the critical value into the second derivative. If the second derivative is positive, then there is a relative minimum there and if the second derivative is negative, then there is a relative maximum there.
The Maple commands that are most useful are the ones for plotting
functions, taking derivatives, and solving equations. By plotting the
function and/or its derivatives, you can get a very good idea of where
it is increasing/decreasing and where it is concave up/concave
down. Then using the solve or fsolve commands
you can find the values of
where
or
. Finally,
you can use Maple to check the signs of
or
in the interior
of the subintervals. The example below shows how you can use Maple to find intervals where the function
is increasing and decreasing.
> f := x-> x^3-3*x+1;
> plot(f(x),x=-3..3);
> solve(D(f)(x)=0,x);
> D(f)(-2);
> D(f)(0);
> D(f)(2);
The plot helps to see how many critical values you have. The solve command shows that there are critical values at
and
which means that the intervals can be broken up into
,
, and
. Remember that if solve doesn't work or doesn't find all critical values, you can use the fsolve command specifying ranges for
in which to solve. Then chosing a point in each interval, we can see that the value of the derivative is positive at
which implies that the function is increasing on the interval
. We can also use the second derivative test to classify
and
as relative maximum or relative minimum. See the Maple commands below to help you do this.
> D[1,1](f)(-1);
> f(-1);
> D[1,1](f)(1);
> f(1);
As you can see, the value of the second derivative at
is negative implying that
is a relative maximum. The value of the second derivative at
is positive which means that
is a relative minimum.
- For the function
- Find ALL critical values and state them in text.
- Use the first derivative test to find intervals on which
is increasing and intervals on which it is decreasing without looking at a plot of the function.
- Without plotting the function
, find all critical points and then classify each point as a relative maximum or a relative minimum using the second derivative test. Be sure to find corresponding
values for each critical value. (You do not need to consider where the derivative is undefined here because there could never be a relative extrema at a point where
is undefined.)
- Find the absolute extrema for the function
on the closed interval
without plotting the function. You may need to plot the derivative of the function to help you find critical values.
Next: About this document ...
Up: lab_template
Previous: lab_template
Dina J. Solitro-Rassias
2010-09-23