- Purpose
- Background
- Introduction
- Definite and indefinite integrals with Maple
- More on computing definite integrals with Maple
- Definite integrals and average values

- Exercises

- Computing net or total distance traveled by a moving object.
- Computing average values, e.g. centroids and centers of mass, moments of inertia, and averages of probability distributions.

Of course, when we use a definite integral to compute an area or a volume, we are adding up area or volume. So you might ask why make a distinction? The answer is that the notion of an integral as a means of computing an area or volume is much more concrete and is easier to understand.

You have learned in class that the definite integral is actually defined as a (complicated) limit of sums, so it makes sense that the integral should be thought of as a sum. You have also learned in class that the indefinite integral, or anti-derivative, can be used to evaluate definite integrals. Students often concentrate on techniques for evaluating integrals, and ignore the definition of the integral as a limit of sums. This is a mistake, for the following reasons.

- Many functions don't have anti-deriviatives that can be written down as formulas. Definite integrals of such function must be done using numerical techniques, which always depend on the definition of the integral as a limit of sums.
- In many applications of the integral in engineering and science, you aren't given the function which is to be integrated and must derive it. The derivation always depends on the notion of the integral as a sum. You will see examples of this later on in the course.

The command to use is shown below.

> int(x^2,x=0..4);

Notice that Maple gives an exact answer, as a fraction. If you want a decimal approximation to an integral, you just put an

> evalf(int(x^2,x=0..4));

To compute an indefinite integral with Maple, you just leave out the range for the limits of integration, as shown below.

> int(x^2,x);

Note that Maple does not include a constant of integration.

You can also use the Maple `int` command with functions or
expressions you have defined in Maple.
For
example, suppose you wanted to find area under the curve of the
function
on the
interval . Then you can define this function in Maple with
the command

> f := x -> x*sin(x);

and then use this definition as shown below.

> int(f(x),x=0..Pi);

You can also simply give the expression corresponding to a label in Maple, and then use that label in subsequent commands as shown below. However, notice the difference between the two methods. You are urged you to choose one or the other, so you don't mix the syntax up.

> p := x*sin(x);

> int(p,x=0..Pi);

you could use the following Maple command.

> int((2*x-3)^5,x=-2..4);

Sometimes you need to compute a definite integral involving a
piecewise-defined function. For example, suppose you have a function
defined as follows

and you needed to compute the definite integral

The best way to do this in Maple is to split it up into two integrals and use the appropriate formula, as shown below. How you split the integral up is determined by where the formula defining the function changes.

> int(2-x^2,x=-5..1)+int(x,x=1..5);

Note that the average value is just a number. Note furthermore that we can rearrange the definition to give

If on , then the average value has the following geometrical interpretation: is the height of a rectangle of width such that the area of this rectangle is equal to the area under the graph of from to . The following example shows you how to compute an average. The last plot command shows the function and the top of this rectangle.

> f :=x -> x*sin(x) ;

> plot(f(x),x=0..Pi);

> f_ave := int(f(x),x=0..Pi)/Pi;

> plot(f(x),f_ave,x=0..Pi);

- For each of the following functions and intervals, determine the
area under the curve using the Maple
`int`command. Include a plot of the curve to verify that it is non-negative over the interval in question.- on .
- on .
- on . Verify that the answer you get is correct by using the formula for the area of a circle.

- Suppose that the flow rate of water, in units of 1000 cubic feet per
hour, over the spillway of a dam
is given by
where is time in hours. The total flow, , over the
spillway from time to is given by

where the units of are thousands of cubic feet.- Compute the total flow over the spillway for the interval , where is the time in hours.
- Compute the average flow rate of water, in units of 1000 cubic feet per hour, over the spillway for the time period .

- Consider the function

Compute the following integral.

2001-03-26