# How and when to use Cauchy's integral formula

1. Jun 3, 2014

### MissP.25_5

Hello.
How do I know when to use Cauchy integral formula. Why do we use the formula in this question? As you can see in my attempt, I got stuck.
Are my values for f(z), z, z​0 here correct?

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2. Jun 3, 2014

### maajdl

You got the right answer, since f(zo)=1.
You use the Cauchy integral when you have to evaluate an integral that matches the pattern of a Cauchy integral!
Sometimes, the contour needs to be closed to get a match and the integral on the closing path can be evaluated by some other means.
The Cauchy integral is very useful in physics and in signal theory.

3. Jun 3, 2014

### MissP.25_5

No, I just solved it and f(z) is not 1, but 1/(z-1).

4. Jun 4, 2014

### maajdl

You proved that your integral is 2 Pi I * the Cauchy integral of f(z)=1 around z=I .
Therefore, your integral is 2 Pi I f(I) = 2 Pi I .

5. Jun 4, 2014

### Ray Vickson

I never look at photo attachments of handwritten work; in fact, if you read the "PF Guidelines" post by vela, you will see that you are not supposed to use them except for very special circumstances---for several good reasons. You should take the trouble to type out your work if you want the helpers to take the trouble to offer free assistance.

6. Jun 4, 2014

### micromass

Staff Emeritus