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Exercise 1073Calculus › Multivariable calculus › Multiple integralsL1Added 3 Nov 2026

Double integral over an annulus

#polar coordinates#annulus#angular integral

Statement

Compute I=∬Ddx dyx2+y2+xy\displaystyle I=\iint_D\frac{dx\,dy}{x^2+y^2+xy} where D={4≤x2+y2≤16}D=\{4\le x^2+y^2\le16\}.

Hint

Polar: x2+y2+xy=r2(1+cos⁡θsin⁡θ)x^2+y^2+xy=r^2(1+\cos\theta\sin\theta).

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Solution

Polar. I=∫02π∫24r dr dθr2(1+cos⁡θsin⁡θ)=∫24drr⋅∫02πdθ1+cos⁡θsin⁡θI=\int_0^{2\pi}\int_2^4\frac{r\,dr\,d\theta}{r^2(1+\cos\theta\sin\theta)}=\int_2^4\frac{dr}{r}\cdot\int_0^{2\pi}\frac{d\theta}{1+\cos\theta\sin\theta}.

Radial. ∫24dr/r=ln⁡2\int_2^4 dr/r=\ln2.

Angular integral. Using cos⁡θsin⁡θ=12sin⁡2θ\cos\theta\sin\theta=\frac12\sin 2\theta and t=θ−π/2t=\theta-\pi/2, substitution u=tan⁡tu=\tan t:
J=∫02πdθ1+12sin⁡2θ=2∫−π/2π/2dt1−12sin⁡2t=2∫−∞+∞du(u−12)2+34=4π3.J=\int_0^{2\pi}\frac{d\theta}{1+\frac12\sin2\theta}=2\int_{-\pi/2}^{\pi/2}\frac{dt}{1-\frac12\sin2t}=2\int_{-\infty}^{+\infty}\frac{du}{(u-\frac12)^2+\frac34}=\frac{4\pi}{\sqrt3}.

Total. I=4πln⁡23I=\frac{4\pi\ln2}{\sqrt3}.

Conclusion. I=4π3ln⁡2I=\frac{4\pi}{\sqrt3}\ln2.