I have read the tutorial of the calculus forum but still dont understand how you are supposed to prove that $\displaystyle \int_0^L \frac{a^2}{(x^2+a^2)^{3/2}} dx = \frac{L}{\sqrt{L^2+a^2}}$ (or rather $\displaystyle \int \frac{a^2}{(x^2+a^2)^{3/2}} dx = \frac{x}{\sqrt{x^2+a^2}}$). I have also looked on the internet and read my analysis-book, but I mostly find examples of rational integrals. What method should I employ? (I tried the substitution $\displaystyle u=\sqrt{x^2+a^2}$ without much success.)