Make the substitution:z = e i θ , d z = i e i θ d θ . z = e^{ iθ} , dz = ie^{iθ}dθ. z = e i θ , d z = i e i θ d θ . Now the complex z z z describes the unit circle C 1 C_1 C 1 in the positive sense as θ θ θ varies from to 2 π . 2\pi. 2 π .
The integral becomes
∫ 0 2 π d θ 2 + sin θ = ∮ C 1 d z i z 2 + z 2 − 1 2 i z \displaystyle\int_{0}^{2\pi}\dfrac{d\theta}{2+\sin \theta}=\oint_{C_1}\dfrac{\dfrac{dz}{iz}}{2+\dfrac{z^2-1}{2iz}} ∫ 0 2 π 2 + sin θ d θ = ∮ C 1 2 + 2 i z z 2 − 1 i z d z
= ∮ C 1 2 d z z 2 + 4 i z − 1 =\oint_{C_1}\dfrac{2dz}{z^2+4iz-1} = ∮ C 1 z 2 + 4 i z − 1 2 d z The poles of the integrand are simple and occur when z 2 + 4 i z − 1 = 0 z^2+4iz-1=0 z 2 + 4 i z − 1 = 0
z ± = − 4 i ± − 16 + 4 2 = ( − 2 ± 3 ) i z_{\pm}=\dfrac{-4i\pm
\sqrt{-16+4}}{2}=(-2\pm\sqrt{3})i z ± = 2 − 4 i ± − 16 + 4 = ( − 2 ± 3 ) i
z + z − = ( − 2 + 3 ) i ( − 2 − 3 ) i = − 1 z_+z_-=(-2+\sqrt{3})i(-2-\sqrt{3})i=-1 z + z − = ( − 2 + 3 ) i ( − 2 − 3 ) i = − 1 Therefore, z + z_+ z + is a (simple) pole inside the unit circle.
Use R e s z = z 0 g ( z ) h ( z ) = g ( z 0 ) h ′ ( z 0 ) , Res_{z=z_0}\dfrac{g(z)}{h(z)}=\dfrac{g(z_0)}{h'(z_0)}, R e s z = z 0 h ( z ) g ( z ) = h ′ ( z 0 ) g ( z 0 ) , where z 0 z_0 z 0 is a simple zero of h ( z ) . h(z). h ( z ) .
Then
R e s ( z + ) = 2 2 z + + 4 i = 1 − 2 i + 3 i + 2 i = 1 3 i Res(z_+)=\dfrac{2}{2z_{+}+4i}=\dfrac{1}{-2i+\sqrt{3}i+2i}=\dfrac{1}{\sqrt{3}i} R es ( z + ) = 2 z + + 4 i 2 = − 2 i + 3 i + 2 i 1 = 3 i 1 Thus, by the residue theorem,
∫ 0 2 π d θ 2 + sin θ = 2 π i ( 1 3 i ) = 2 π 3 \displaystyle\int_{0}^{2\pi}\dfrac{d\theta}{2+\sin \theta}=2\pi i(\dfrac{1}{\sqrt{3}i})=\dfrac{2\pi}{\sqrt{3}} ∫ 0 2 π 2 + sin θ d θ = 2 πi ( 3 i 1 ) = 3 2 π
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