# Variable change in definite integration

1. Mar 11, 2014

### bartrocs

I am having trouble understanding how the author of the textbook "Physics for scientists and engineers" (Randall D Knight), is performing the variable change in the definite integral of the picture that I have taken a screen clipping of. Can someone please help me figure this out?

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2. Mar 11, 2014

### grzz

Variable r is being replaced by variable u where

u = z$^{2}$ + r$^{2}$.

Hence

$\frac{du}{dr}$ = 2r

since z is a constant.

3. Mar 11, 2014

### bartrocs

thanks! I understand now