Evaluate $\int_0^{\infty} \frac{\sqrt{x}}{(x+1)^2} {\rm d}x$

complex-analysiscontour-integrationimproper-integrals

I am trying to evaluate
\begin{align}
\int_0^{\infty} \frac{\sqrt{x}}{(x+1)^2}{\rm d}x = \frac{\pi}{2}
\end{align}

(This comes from Mathematica)

First of all, I want to solve this problem with the Residue theorem.

Assuming $\sqrt{z}$ be the principal branch, $ \operatorname{Res}[f;-1] = -\frac{i}{2}$. But I am having trouble with integral range.

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If I neglect (this should be checked) the contribution from $C_R$ and $C_{\epsilon}$, then this gives $\int_{-\infty}^{\infty}$ and dividing half I can fit result but the integrand is not even function…

Best Answer

It helps to first write $y=\sqrt{x}$ so your integral is $\int_0^\infty\frac{2y^2}{(y^2+1)^2}dy=\int_{-\infty}^\infty\frac{y^2}{(y^2+1)^2}dy$. We can evaluate this integral over $\Bbb R$ with the residue theorem as$$2\pi i\lim_{y\to i}\frac{d}{dy}\frac{y^2}{(y+i)^2}=2\pi i\lim_{y\to i}\frac{2iy}{(y+i)^3}=\frac{2\pi i2i^2}{(2i)^3}=\frac{\pi}{2}.$$If you're unconfident with the residue theorem, it helps to double-check this with a real method, by setting $y=\tan t$ in the integral on $\Bbb R^+$. The integral is then$$\int_0^{\pi/2}2\sin^2tdt=\frac{\pi}{2}2\times\frac12=\frac{\pi}{2}.$$