# Math Help - Without using Integral by Part

2. ## Re: Without using integral by parts

$\displaystyle\int_0^{\infty}e^{-t}\arctan t\;dt=\int_0^{\infty}\int_0^t \frac{e^{-t}}{1+x^2} \;dx\;dt$

Now switch the order of integration:

$\qquad\qquad=\int_0^{\infty}\int_x^{\infty} \frac{e^{-t}}{1+x^2} \;dt\;dx=\int_0^{\infty}\frac{e^{-x}}{1+x^2}\;dx$

3. ## Re: Without using integral by parts

Originally Posted by LordoftheFlies
$\displaystyle\int_0^{\infty}e^{-t}\arctan t\;dt=\int_0^{\infty}\int_0^t \frac{e^{-t}}{1+x^2} \;dx\;dt$

Now switch the order of integration:

$\qquad\qquad=\int_0^{\infty}\int_x^{\infty} \frac{e^{-t}}{1+x^2} \;dt\;dx=\int_0^{\infty}\frac{e^{-x}}{1+x^2}\;dx$

Can u explain that step?

4. ## Re: Without using Integral by Part

It's a standard trick. Watch this video, he'll explain the basic idea better than I can.

Spoiler: