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Evaluate \( \displaystyle\oint_{|z|=1} \frac{dz}{z} \).

  • A.\( 2\pi i \)
  • B.\( 0 \)
  • C.\( 2\pi \)
  • D.\( \pi i \)

Answer: A

Put \( z = e^{i\theta} \), so \( dz = ie^{i\theta}d\theta \):

\[ \oint_{|z|=1}\frac{dz}{z} \;=\; \int_{0}^{2\pi} \frac{ie^{i\theta}}{e^{i\theta}}\,d\theta \;=\; i\int_{0}^{2\pi} d\theta \;=\; 2\pi i. \]

It is non-zero because the origin, where the integrand fails to be analytic, lies inside the contour

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