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Question

Given $f(z) = \frac{1}{z+1} - \frac{2}{z+3}$. If $C$ is a counterclockwise path in the $z$-plane such that $|z+1|=1$, the value of $\frac{1}{2\pi j}\oint_C f(z)dz$ is

The correct answer is
1

To evaluate the integral \(\frac{1}{2\pi j}\oint_C f(z)dz\) where f(z) = \frac{1}{z+1} - \frac{2}{z+3} and C is the path |z+1|=1, we can use the residue theorem from complex analysis.

The residue theorem states that if you have a function f(z) analytic inside and on some simple closed contour C, except for isolated singularities, then:

\[\frac{1}{2\pi j}\oint_C f(z) dz = \sum \text{Res}(f, z_k)\]

where the sum is over all residues inside the contour C.

Step-by-step Solution:

  1. Identify singularities of f(z):
    • The function \frac{1}{z+1} has a singularity at z = -1.
    • The function \frac{2}{z+3} has a singularity at z = -3.
  2. Determine which singularities lie inside the contour |z+1|=1:
    • The circle |z+1|=1 is centered at z = -1 with a radius of 1, so it will include z = -1 but not z = -3.
  3. Calculate the residue at z = -1:
    • The function \frac{1}{z+1} has a residue of 1 at z = -1.
    • The function \frac{2}{z+3} does not contribute since z = -3 is not inside the contour.
    • Thus, the total residue inside the contour is simply 1.
  4. Apply the residue theorem:
    • The value of the integral is the sum of the residues inside the contour times 2\pi j, thus:
    • \(\frac{1}{2\pi j}\oint_C f(z)dz = 1\).

After evaluating the integral, the correct answer is 1.

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Important Questions from Complex Variables

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  2. \(\cos \frac{\pi}{3}+\frac{1}{2} \cos \frac{2 \pi}{3}\)\(\frac{1}{3} \cos \frac{3 \pi}{3} \ldots \infty\)  = will 
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  4. The modulus of 1 + cos α + i sin α is

  5. Given \(f(z)=\frac{1}{z+1}-\frac{2}{z+3}\). If C is a counterclockwise path in the z-plane such that |z + 1| = 1, the value of \(\frac{1}{2\pi i}\int_c f(z)dz\) is

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