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May 18th, 2014, 03:42 PM   #1
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double integral problem

Hi,
Trying to solve the attached double integral, not sure if I have done it correctly, can someone check? (see attachment).

thanks
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May 18th, 2014, 07:06 PM   #2
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I get almost the same result, you've just made an error in the last step.
$$r^2 + 1 \ne 0 \; \text{where $r = 0$}$$
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May 18th, 2014, 08:13 PM   #3
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thanks for checking, could please show me your working at the point where i have made a mistake.

thanks for your time.
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May 18th, 2014, 08:25 PM   #4
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I worked the integral slightly differently, but your error isn't in the integral.

You have
$$I = \left. \frac{\pi}{4}\left[ \left(r^2 + 1\right) \ln{\left( r^2 + 1 \right)} - \left( r^2 + 1 \right) \right] \right|_0^3$$
Which I agree with. But the next line (the last one is wrong). I'm reasonably sure that you evaluated the expression at $r = 3$ correctly, but I think you got 0 at $r = 0$, which is not correct.
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May 18th, 2014, 08:35 PM   #5
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yes you're right, thanks for your help. I get 11.02 now.

Last edited by harley05; May 18th, 2014 at 08:39 PM.
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May 18th, 2014, 08:51 PM   #6
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Me too.
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