# Integration by change of variable

1. Jun 27, 2008

### asif zaidi

Hello:

In Problem Solution part below, I am not sure of Step 2 and am having problems with Step 3.

Problem Statement:

Let D = {(x1,x2)| x1,x2>0, 1<= x1$$^{2}$$ - x2$$^{2}$$ <=9, 2 <= x1x2 <= 4.
Use hyperbolic coordinates g(u,v) = {u$$^{2}$$ - v$$^{2}$$, uv} to show that

integ on D of (x1$$^{2}$$ + x2$$^{2}$$) dx1dx2 = 8 ---- equation 1.

Problem Solution:

Step1: To show that Jacobian of g is non-zero
This was OK. Det is 2u$$^{2}$$ + 2v$$^{2}$$. it is also given that u,v >0. Therefore det will always be >0

Step2: To convert x1,x2 into u, v

Substituting (u$$^{2}$$ - v$$^{2}$$)$$^{2}$$ into x1$$^{2}$$ and (uv)$$^{2}$$ into x2$$^{2}$$ equation 1 changes to after manipulation to ( I also did multiply by the Jacobian)

integ on R of (2u$$^{6}$$ + 2v$$^{6}$$

Step3: To determine limits of u,v.
This is where I am having problems with

How do I translate 1<= x1^2 - x2^2 <=9 and 2 <= x1x2 <= 4 into u, v. Everything I do doesn't give me the ans in the question.

Thanks

Asif
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Jun 28, 2008

### HallsofIvy

Staff Emeritus
You are taking x1= u2- v2 and x2= uv?

It is far simpler to let u= x12- x22 and v= x1x2.

3. Jun 29, 2008

### asif zaidi

Excellent - thank you.
It worked.