Solving a Double Integration Problem with Unknown Variables: How to Proceed?

  • Thread starter chetzread
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In summary: Yes, an integrand involving x, y, and u is wrong. The integrand must involve only x and y. You must rewrite u in terms of x and y.In summary, the original problem involves expressing all the variables, x and y, in terms of u. The attempt at a solution involved letting u = x^2 + y^2 and attempting to switch the order of integration, but the integrand involved all three variables. The correct approach would be to find a different expression for u in terms of x and y, possibly by using a circle and choosing a more appropriate variable.
  • #36
@chetzread, I believe that BvU's hint is not about merely switching the order of integration (i.e., dxdy vs. dydx), but instead, about changing to a polar integral. This problem quite a bit easier if you do it this way.
 
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  • #37
chetzread said:
It's cos theta, so?
So ? Wouldn't substituting ##\theta## as integration variable be a sensible thing to try ?
 
  • #38
BvU said:
So ? Wouldn't substituting ##\theta## as integration variable be a sensible thing to try ?
ok, i got the ans now ...I'm wondering is this a special case? I have never learned and see this before ...
 
  • #39
Humor me and tell me what you did ...
 
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  • #40
BvU said:
Humor me and tell me what you did ...
？
 
  • #41
You said
chetzread said:
ok, i got the ans now
could you share your working with us ?
 

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