in the picture you can see the problem, i did a little bit of the way to solve it but i stuck now.
hope you can help
thanks.
Another way to do it if you can't remember the formulas is through trig substitution. Draw a right triangle such that $\displaystyle \displaystyle \tan( \theta) = \left( \frac{x}{\sqrt{2}} \right)$. Find $\displaystyle d \theta$ in terms of x then substitute everything. You'll be left with a nice cancellation and an easy integral. Then back substitute. You'll get the same answer that TheCoffeeMachine is leading you to.