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Claire84
Apr30-04, 01:03 PM
I was wondering if someone could give me a hand here with 2b) on the following link.
http://www.am.qub.ac.uk/users/j.mccann/teaching/ama102/2003/assignments/assign_8.pdf
For part a) I got it to be equal to 3x^2+3y^2+3z^2+2y-2xy, and I'm hoping that's right!

However, for part b) I can't seem to get the answer they're after at all. We haven't covered any examples of this in our lectures yet and we won't have any lectures before the homework has to be handed in due to May Day, so any help would be much appreciated. Just to make sure I started it off right, could you just check if this is correct or not (sorry, I'm no good with LaTex!)-

triple integral of (3r^2sin^2(theta)cos^2(f)+ 3r^2sin^3(theta)sin^2(f) + 3r^2cos^2(theta) + 2rsin(theta)sin(f) - 2r^2sin^2(theta)cos(f)sin)f))r^2sin(theta)drdfdthe ta

where the integral with respect to r is within the limits 0 and a, with respect to f is 0 to 2pi and with respect to theta is 0 to pi (where f is the asimuthal angle or whatever it's called). I'd really appreciate if you could just ehck I've done that bit okay, because it'd be a bit pointless me running through it if the mistake was in the first line.

Btw, the answer I get at the end has an 8 on the numerator instead of a 12, so the answer isn't completely far out so I'm hoping it's just a wee mistake somewhere. Thanks! :smile:


Btw, sorry if this is posted in the wrong forum!

arildno
Apr30-04, 01:28 PM
The divergence should be 3r^(2). I'll look a bit further into this..

arildno
Apr30-04, 01:32 PM
Ok:
If you go back to your vector field, you'll see that from both the j'th and k'th component, you will gain a 2xy term, but with opposite signs.
In your original expression, you've ended up with 2y-2xy instead

Claire84
Apr30-04, 02:45 PM
Ah so shold it look like this then-

3x^2 + 3y^2 + 3z^2 ?

Gah I can be so stupid with even the simplest things!

Thanks for helping btw! :smile:

arildno
Apr30-04, 02:51 PM
That's what I got, too (3r^(2)).

Claire84
May2-04, 10:45 AM
Btw, for part 3a), that's a mistake, right? I mean I keep getting xi - yj + (-y^2 - x)k and I asked one of the phd students about it and they got the same but I just want to check before I email the lecturer about it.......

arildno
May2-04, 10:50 AM
Agreed, he'll be writhing in shame..

Claire84
May2-04, 10:52 AM
Hope that wasn't sarcasm there. :tongue: