Question #286644

Using Residue theorem evaluate ∫0^2π dθ/2+sinθ ?


1
Expert's answer
2022-01-14T09:41:38-0500

Make the substitution:z=eiθ,dz=ieiθdθ.z = e^{ iθ} , dz = ie^{iθ}dθ. Now the complex zz describes the unit circle C1C_1 in the positive sense as θθ varies from to 2π.2\pi.

The integral becomes

02πdθ2+sinθ=C1dziz2+z212iz\displaystyle\int_{0}^{2\pi}\dfrac{d\theta}{2+\sin \theta}=\oint_{C_1}\dfrac{\dfrac{dz}{iz}}{2+\dfrac{z^2-1}{2iz}}

=C12dzz2+4iz1=\oint_{C_1}\dfrac{2dz}{z^2+4iz-1}

The poles of the integrand are simple and occur when z2+4iz1=0z^2+4iz-1=0


z±=4i±16+42=(2±3)iz_{\pm}=\dfrac{-4i\pm \sqrt{-16+4}}{2}=(-2\pm\sqrt{3})i

z+z=(2+3)i(23)i=1z_+z_-=(-2+\sqrt{3})i(-2-\sqrt{3})i=-1

Therefore, z+z_+ is a (simple) pole inside the unit circle.

Use Resz=z0g(z)h(z)=g(z0)h(z0),Res_{z=z_0}\dfrac{g(z)}{h(z)}=\dfrac{g(z_0)}{h'(z_0)}, where z0z_0 is a simple zero of h(z).h(z).

Then

Res(z+)=22z++4i=12i+3i+2i=13iRes(z_+)=\dfrac{2}{2z_{+}+4i}=\dfrac{1}{-2i+\sqrt{3}i+2i}=\dfrac{1}{\sqrt{3}i}

Thus, by the residue theorem,


02πdθ2+sinθ=2πi(13i)=2π3\displaystyle\int_{0}^{2\pi}\dfrac{d\theta}{2+\sin \theta}=2\pi i(\dfrac{1}{\sqrt{3}i})=\dfrac{2\pi}{\sqrt{3}}



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