Exercise · ZW-REAL-INTEGRAL-BY-RESIDUES-003 · ★☆☆ Basic
For \(\int_{-\infty}^{\infty}\frac{dx}{x^{2}+9}\), which poles of \(1/(z^2+9)\) enter the sum of residues?
Choose one option.
Show answer
- A. Only \(3i\)
Explanation
We close in the upper half-plane, which contains \(3i\) but not \(-3i\). The result is \(2\pi i\cdot\frac{1}{6i}=\pi/3\).
Review the concept: Real integral by residues
A method to compute \(\int_{-\infty}^{\infty} f(x)\,dx\) with \(f\) rational: close the real axis with a semicircle in the upper half-plane and apply the residue theorem.
\[\int_{-\infty}^{\infty} f(x)\,dx=2\pi i\sum_{\operatorname{Im}z_k>0}\operatorname{Res}(f,z_k)\]
More practice problems: real integral by residues
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- ZW-REAL-INTEGRAL-BY-RESIDUES-004 Compute Res\!((1)/(z^2+1), i).
- ZW-REAL-INTEGRAL-BY-RESIDUES-005 Compute ∫_-∞^∞(dx)/(x^2+2x+2).
- ZW-REAL-INTEGRAL-BY-RESIDUES-006 Compute ∫_-∞^∞(dx)/((x^2+1)^2).
- ZW-REAL-INTEGRAL-BY-RESIDUES-007 Compute ∫_-∞^∞(dx)/((x^2+1)(x^2+4)).
- ZW-REAL-INTEGRAL-BY-RESIDUES-008 Compute ∫_-∞^∞(dx)/(x^4+1).
- ZW-REAL-INTEGRAL-BY-RESIDUES-009 Let f=P/Q be rational with no real poles. Which condition guarantees that the integral over the sem…
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