Use residue calculus to compute the integral ∫∞−∞1(z2+25)(z2+16)dz
My solution
If we add to the interval IR=[−R,R] add the semicircle γR in the upper half plane with centre at the origin and radius R, then we obtain a closed contour ΓR (which we consider to be oriented in the positive direction over which the integral f(z)=1(z2+25)(z2+16) can be computed using the Cauchy's Residue theorem.
∫∞−∞1(z2+25)(z2+16)dz=lim
In this case, we have two simple poles at the points z_{1}=4i and z_{2}=5i. The residues are computed as
\begin{align} \text{Res}(z_{1})&=\frac{1}{(z^{2}+25)D(z^{2}+16)}\big|_{z=4i} \\ &=\frac{1}{(z^{2}+25)2z}\big|_{z=4i} \\ &=\frac{1}{9\cdot8i} \\ &=\frac{1}{72i} \\ &=-\frac{i}{72} \end{align}
Similarly, we get for z_{2} that
\begin{align} \text{Res}(z_{2})&=\frac{1}{D(z^{2}+25)(z^{2}+16)}\big|_{z=5i} \\ &=\frac{1}{2z(z^{2}+16)}\big|_{z=5i} \\ &=\frac{1}{10i \cdot (-9)} \\ &=-\frac{1}{90i} \\ &=\frac{i}{90} \end{align}
Thus, we get
\begin{align} \int_{-\infty}^{\infty}\frac{1}{(z^{2}+25)(z^{2}+16)}dz &=2\pi i \left(\text{Res}(z_{1})+\text{Res}(z_{2})\right) \\ &=2 \pi i \left(-\frac{i}{360}\right) \\ &=\frac{\pi}{180} \end{align}
Are there other methods one could use to arrive at this answer?
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