Complex City getting bigger

Calculus Level 3

c z 3 6 3 z i d z \large \oint_c \frac{z^3-6}{3z-i} \, dz

Evaluate the integral above, if it is taken in clockwise direction.

Hint : Use Cauchy Integral.

1 π 8 6 i π \frac \pi8 - 6i\pi 4 81 π 6 i π \frac4{81}\pi - 6i \pi 2 81 π 4 i π \frac2{81}\pi - 4i \pi

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1 solution

Tom Engelsman
Oct 25, 2017

The Cauchy Integral takes on the form 1 2 π i C f ( z ) z a d z \frac{1}{2\pi i} \cdot \oint_C \frac{f(z)}{z - a} dz , where C C is the unit circle centered at the origin of the complex plane. The above complex integrand can be rewritten as:

z 3 6 3 z i = z 3 6 3 ( z i / 3 ) = z 3 / 3 2 z i / 3 \frac{z^3 - 6}{3z - i} = \frac{z^3 - 6}{3(z - i/3)} = \frac{z^{3}/3 - 2}{z - i/3}

The pole z = i 3 z = \frac{i}{3} is contained within C C and thus the Cauchy Integral computes to 2 i π f ( a ) = 2 i π f ( i / 3 ) = 2 i π ( 1 3 ( i 3 ) 3 2 ) = 2 π 81 4 i π . 2i\pi f(a) = 2i\pi f(i/3) = 2i\pi \cdot (\frac{1}{3} \cdot (\frac{i}{3})^3 - 2) = \boxed{\frac{2\pi}{81} - 4i\pi}.

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