# Math Help - definite integral #5

1. ## definite integral #5

Show that $\int_{0}^{1} \frac{2a^{2}}{a^{2}+x^{2}} \frac{\arctan (\sqrt{2a^{2}+x^{2}})}{\sqrt{2a^{2}+x^{2}}} \ dx = \pi \arctan \Big(\frac{1}{\sqrt{2a^{2}+1}} \Big) - \Big( \arctan \frac{1}{a} \Big)^2$

If you let $a = 1$, you'll get a integral that maybe you've seen before.

2. ## Re: definite integral #5

I'll offer a starting point.

Let p and q be polynomials. Then $\frac{1}{p(p+q)}} + \frac{1}{q(p+q)}} = \frac{1}{pq}$

Let $p = a^{2}+x^{2}$ and $q = a^{2}+t^{2}$

Then $\frac{1}{(a^{2}+x^{2})(2a^{2}+x^{2}+t^{2})} + \frac{1}{(a^{2}+t^{2})(2a^{2}+x^{2}+t^{2})} = \frac{1}{(a^{2}+x^{2})(a^{2}+t^{2})}$

Now integrate both sides over the unit square centered at $(\frac{1}{2}, \frac{1}{2})$

$\int^{1}_{0} \int^{1}_{0} \frac{dx \ dt}{(a^{2}+x^{2})(2a^{2}+x^{2}+t^{2})} + \int^{1}_{0} \int^{1}_{0} \frac{dx \ dt}{(a^{2}+t^{2})(2a^{2}+x^{2}+t^{2})} = \int^{1}_{0} \int_{0}^{1} \frac{dx \ dt}{(a^{2}+x^{2})(a^{2}+t^{2})}$

If you switch the order of integration of the second integral on the left, which is permissible since the integrand is never negative, you'll notice that the the two integrals on the left are identical.

$2 \int^{1}_{0} \int^{1}_{0} \frac{dx \ dt}{(a^{2}+x^{2})(2a^{2}+ x^{2}+t^{2})} = \int^{1}_{0} \frac{dx}{a^{2}+x^{2}} \int_{0}^{1} \frac{dt}{a^{2}+t^{2}}$

3. ## Re: definite integral #5

$\frac{\frac{\pi }{2}-\arctan \left( \sqrt{2{{a}^{2}}+{{x}^{2}}} \right)}{\sqrt{2{{a}^{2}}+{{x}^{2}}}}=\int_{0}^{1} {\frac{dt}{2{{a}^{2}}+{{x}^{2}}+{{t}^{2}}}},$ so we need to compute $\int_{0}^{1}{\int_{0}^{1}{\frac{dx\,dt}{\left( {{a}^{2}}+{{x}^{2}} \right)\left( 2{{a}^{2}}+{{x}^{2}}+{{t}^{2}} \right)}}}.$