Integration by parts is another important technique of integration. You might ask, when should I use it? The general rule is, whenever you have a product of two functions, one that you can integrate, and one that you can differentiate. In the context of other integration techniques, generally speaking, you should always use integration by substitution first if you can. If you cannot, then consider integration by parts.
The formula for integration by parts is:
\(\int u \, dv=uv-\int v \, du\)
The critical step is to correctly identify
\(u\)
and
\(dv\)
. Let’s look at an example in the next section.
An Example of Integration by Parts
Let's find the integral of the following function:
\(\int x{e}^{x} \, dx\)
If you were to try integration by substitution first (which you should), you would have seen that it did not work. Now, let us try integration by parts. The dilemma arises: what should we pick as
\(u\)
and
\(dv\)
in the following formula?
\(\int u \, dv=uv-\int v \, du\)
One thing for sure is, we want to pick
\(dv\)
to be something that we can integrate.
\({e}^{x}\)
would be a good candidate, since the integral of
\({e}^{x}\)
is itself. Let's try the following:
\(u=x\)
,
\(dv={e}^{x}\)
,
\(du=dx\)
and
\(v={e}^{x}\)
. This gives:
\(x{e}^{x}-\int {e}^{x} \, dx\)
This looks good, since we are now left with integrating
\({e}^{x}\)
, which we know how to do. It is simply
\({e}^{x}\)
. Finally, do not forget to add the constant
\(C\)
. This produces the final answer:
\(x{e}^{x}-{e}^{x}+C\)
We are now done. Note that if you picked the wrong
\(u\)
and
\(dv\)
on the first try, don't worry. You would have seen that it did not work out, and you would just have to pick another pair. With enough practice, you will be able to make the right choices within the first couple tries.
What's Next
Best bet? Start with the practice problems at the top of this page to get more familiar with integration by parts. Want to dig deeper? Sign up for Cymath Plus today! You can also check out the Cymath app for iOS and Android.
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