Now that we have used integration by parts successfully to evaluate
indefinite integrals , we turn our attention to definite integrals. The integration technique is really the same, only we add a step to evaluate the integral at the upper and lower limits of integration.
Integration by parts for definite integrals
Let
and
be functions with continuous derivatives on
Then
Finding the area of a region
Find the area of the region bounded above by the graph of
and below by the
-axis over the interval
This region is shown in
[link] . To find the area, we must evaluate
For this integral, let’s choose
and
thereby making
and
After applying the integration-by-parts formula (
[link] ) we obtain
Use
u -substitution to obtain
Thus,
At this point it might not be a bad idea to do a “reality check” on the reasonableness of our solution. Since
and from
[link] we expect our area to be slightly less than 0.5, this solution appears to be reasonable.
Find the volume of the solid obtained by revolving the region bounded by the graph of
the
x -axis, the
y -axis, and the line
about the
y -axis.
The best option to solving this problem is to use the shell method. Begin by sketching the region to be revolved, along with a typical rectangle (see the following graph).
To find the volume using shells, we must evaluate
To do this, let
and
These choices lead to
and
Substituting into
[link] , we obtain
The integration-by-parts formula allows the exchange of one integral for another, possibly easier, integral.
Integration by parts applies to both definite and indefinite integrals.
Key equations
Integration by parts formula
Integration by parts for definite integrals
In using the technique of integration by parts, you must carefully choose which expression is
u. For each of the following problems, use the guidelines in this section to choose
u. Do
not evaluate the integrals.