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**1. Homework Statement**

[tex]\int_0^a {x\sqrt{x^2+a^2}\,dx}[/tex]

Also, (A>0)

**3. The Attempt at a Solution**

Firstly, I set

u=x^2+a^2

Then take the derivative,

du=2x dx

[tex]1/2\int_0^a {\sqrt{u}\,du}[/tex]

Now I integrated. So

(1/3) * [(x^2+a^2)^3/2] from a to 0.

I ended up with

(1/3)[(a^2+a^2)^(3/2)-a^3]

This is where I get lost. It must have something to do with the (A>0). The answer in the book is:

(1/3)(2*sqrt(2)-1)a^3

I can't see how to eliminate the a's to get the (2*sqrt(2)-1).

Thanks for the help.