# Integration By Parts

I understand this integration technique, for the most part. One thing I am curious to know is why, when you do your rudimentary substitution for this particular technique, does dv have to always include dx?

## Answers and Replies

HallsofIvy
$\int udv= \int d(uv)- \int vdu$. Of course, $\int d(uv)= uv$.