Need help finding inverse transformation for Jacobian

  • Thread starter Samuelb88
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  • #1
Samuelb88
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Homework Statement


Suppose R is a plane region bounded by xy=1, xy=3, x^2-y^2=1, x^2-y^2= 4. Use the substitution u=xy, v=x^2-y^2 to evaluate

[tex]I = \iint\limts_R \, (x^2+y^2) dx\,dy[/tex]

The Attempt at a Solution


Using the substitutions given, I find R` = { (u,v) | u for all [1,3] ; v for all [1,4] }

So after this, it would seem I need to find the inverse transformation so I can make an appropriate substitution for the integrand and evaluate the jacobian so I can integrate over R`. However, I am having difficulties finding an explicit equation for both x and y in terms u and v. Here's what I've managed to do so far although its ultimately nothing... :(

Squaring both sides of the equations relating u and v to x, y, I find:

(I) [tex]u^2 = x^2y^2[/tex]
(II) [tex]v^2 = x^4 - 2x^2y^2+y^4[/tex]

Adding equations (I) + (II) I find:

(III) [tex] v^2 + 2u^2 = x^4 +y^4[/tex]

Dividing (II)/(I) equations I find:

(IV) [tex] \frac{v^2}{u^2}\right) +2 = \frac{x^4+y^4}{x^2y^2}[/tex]

And dividing equations (IV)/(III) I ultimately find:

[tex]x^4+y^4=2(\frac{u^2v^2+2u^4}{v^2+2u^2}\right) )+ v^2[/tex]

So as I said above, I really managed to find nothing at all. I've been fiddling around with both equations for a while now in hopes of finding a desirable "form" to be able to explicitly define both x and y in terms of u and v, but I am totally stumped... Any advice or ideas would be much appreciated. :)

Thanks
 

Answers and Replies

  • #2
Dick
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What's v^2+4*u^2? You don't have to express x and y in terms of u and v. You just have to express x^2+y^2 in terms of u and v.
 
Last edited:
  • #3
Samuelb88
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So then how should I calculate the jacobian? I thought you needed x(u,v), y(u,v) to do so.
 
  • #4
Dick
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You can get that jacobian by calculating the jacobian of u(x,y) and v(x,y) and inverting it. Can't you?
 

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