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contour integral...

could someone please verify that my answer is correct to this as i`m not sure:

Evaluate Cdz\int_{C} dz

where C: x236+y24=1\displaystyle \frac{x^{2}}{36}+\frac{y^{2}}{4}=1

from z=2iz=2i to z=2iz=-2i

my solution:

parametrized z curve: z(t)=6cos(t)+2sin(t)iz(t)=6 \cos(t)+2 \sin(t)i

so
Unparseable latex formula:

\diaplaystyle dz=(-6 \sin(t)+2 \cos(t)i)dt, \frac{\pi}{2} \leq t \leq\frac{3 \pi}{2}



my answer is 4 \displaystyle -4

am i correct? or would i get a different answer (the correct one) by splitting the left part of the ellispe into an upper then lower curve, and integrating along each successive curve?
(edited 9 years ago)
Reply 1
Original post by Hasufel
could someone please verify that my answer is correct to this as i`m not sure:

Evaluate Cdz\int_{C} dz

where C: x236+y24=1\displaystyle \frac{x^{2}}{36}+\frac{y^{2}}{4}=1

from z=2iz=2i to z=2iz=-2i

my solution:

parametrized z curve: z(t)=6cos(t)+2sin(t)iz(t)=6 \cos(t)+2 \sin(t)i

so
Unparseable latex formula:

\diaplaystyle dz=(-6 \sin(t)+2 \cos(t))dt, \frac{\pi}{2} \leq t \leq\frac{3 \pi}{2}



my answer is 4 \displaystyle -4

am i correct? or would i get a different answer (the correct one) by splitting the left part of the ellispe into an upper then lower curve, and integrating along each successive curve?


What's happened to the 'i' in your dz?
Reply 2
Original post by davros
What's happened to the 'i' in your dz?


c**p! sorry! - i just forgot to include it in my post - sorted now, though.

Am i correct?
Hasufel
..
Your answer is wrong. If you correct for not having the 'i' in dz, you should get the right answer, but from your previous post I'm not sure if you did have the 'i' in dz, you just didn't put it in the post. (In which case you will need to post your working - as your current answer is wrong).
Reply 4
Original post by DFranklin
Your answer is wrong. If you correct for not having the 'i' in dz, you should get the right answer, but from your previous post I'm not sure if you did have the 'i' in dz, you just didn't put it in the post. (In which case you will need to post your working - as your current answer is wrong).




Thanks! - think i just made a simple mistake - in splitting the integral into real and imaginary parts, i forgot to include the "i" in the cos one, and just wrote:

2π23π2cos(t)dt\displaystyle 2 \int_{\frac{ \pi}{2}}^{\frac{3 \pi}{2}} \cos(t)dt !
(edited 9 years ago)
Hasufel
..
So, now that you've got the right answer, note that if this was a "normal" (not complex) integral, you'd go:

abdz=[z]ab=ba\displaystyle \int_a^b \, dz = [z]_a^b = b - a, where a, b are your start/end points, respectively.

Spoiler

Reply 6
Original post by DFranklin
So, now that you've got the right answer, note that if this was a "normal" (not complex) integral, you'd go:

abdz=[z]ab=ba\displaystyle \int_a^b \, dz = [z]_a^b = b - a, where a, b are your start/end points, respectively.

Spoiler



Thanks for your insight! - i feel better about how to do these sorts of Q`s now!

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