# Help with change of variables please

1. Mar 4, 2005

I'm trying to evaluate the double integral

$$\int \int \sqrt{x^2 + y^2} \, dA$$ over the region R = [0,1] x [0,1]
using change of variables

Now I know polar coordinates would be the most efficient way, and thus I could say r= $$\sqrt{x^2 + y^2}$$ . Is this legal to use polar coordinates when doing change of variables? So if I do it this way, would I integrate from theta goes from 0 to 2pi and then r goes from 0 to 1?
I'm utterly confused. Anyone able to help me out a bit? Thanks

2. Mar 4, 2005

or would I just set u = x^2 and v = y^2 ....and then take the jacobian of the transformation and then set up the integral?

3. Mar 4, 2005

lol dang...must be a slow day around here? :(

Well, after fooling around, I've got an answer. I set u = x^2, v =y^2, and then calculated the jacobian of T which was 1. The image transformation limits of integration for u and v turned out to be the same [0,1] x [0,1]

So I did the following calculation (both integrals going from 0 to 1)

$$\int \int \sqrt{u + v} * (1) dudv$$

which resulted in a value of roughly 3.238.

4. Mar 4, 2005

### HallsofIvy

I doubt that you will find that polar coordinats ARE particularly suitable. The crucial point is that the area you are integrating over is a square, not a disk.

It's not all that difficult to integrate in x,y. Integrating first with respect to y, let
y/x= tan(&theta;) Then x2+ y2= x2(1+ (y/x)2)= x2(1+ tan2(x))= x2sec2(\theta) so that
$$\sqrt{x2+ y2}= xsec(\theta)$$

5. Mar 4, 2005

Thanks very much HallsofIvy for your reply. I would have simply integrated with respect to x and y, but I specifically have to integrate using change of variables. I did so, as I mentioned above, by doing the following:

I set u = x^2, v =y^2, and then calculated the jacobian of T which was 1. The image transformation limits of integration for u and v turned out to be the same [0,1] x [0,1]

So I did the following calculation (both integrals going from 0 to 1)

$$\int \int \sqrt{u + v} * (1) dudv$$

which resulted in a value of roughly 3.238.

Turned about to be actually a much cleaner calculation than simply doing it with respect to x and y, but I just wondered if I got the correct answer.

Does my logic and answer seem sound here? Thanks again

6. Mar 4, 2005

### hypermorphism

Hi Ninja,
Two things, the first is that you should be aware your transformation is not globally invertible; however, it does happen to be invertible over your region of integration. The next thing is that the Jacobian for your integral is not 1, it is the messy $$dx\wedge dy = d(\sqrt{u})\wedge d(\sqrt{v}) = \frac{du}{2\sqrt{u}}\wedge\frac{dv}{2\sqrt{v}} = \frac{du dv}{4\sqrt{uv}}$$ which makes the integral much more complicated.
Getting your integral in polar coordinates is actually not that difficult (just refer to the trigonometry of the situation, breaking the square into two triangles). Integrating in polar coordinates, I get something much less than 3.238.

Last edited: Mar 4, 2005
7. Mar 4, 2005

Thanks very much hypermorphism, and THANKS for catching my fatal flaw with the Jacobian computation...wow was I ever off. I'm just completely lost at this point. I don't really know what you mean by splitting the square into two triangles...I mean I literally know what you're talking about but I don't see how that would help any with doing polar coordinates.

I also have no clue what to do as far as polar coordinates...do you mean, can I just take

r= $$\sqrt{x^2 + y^2}$$ ? If so, how would I integrate ?

$$\int \int \ r * |jacobian| r drd \theta$$ And the area of integration would be from 0 to 2pi and then....

ah geez I'm completely lost and desperate for help...any pointers would be greatly appreciate thanks!

Last edited: Mar 4, 2005
8. Mar 4, 2005

### dextercioby

It can't be done in polar coordinates,as Halls explained and as it is pretty obvious by a drawing,the square [0,1]*[0,1] is not a circle of radius 1...

Daniel.

9. Mar 4, 2005

### hypermorphism

Draw the square [0,1]x[0,1]. The radius' lower bound for our integral is 0 so we needn't worry anymore about it. The radius' upper bound is going to sweep from (let t=theta) t=0 to t=pi/2. Note that there is a break at t=pi/4 (when the radius is the diagonal of the square). So we shall split our integral at that break and sum the areas of the resulting two triangles.
First we do the lower triangle. Draw a sample radius from the origin to a point on the boundary of the lower triangle. You get a right triangle, and get the relation cos(t) = 1/r, or r = sec(t). For our lower triangle, we then have the integral:
$$\int_0^{\frac{\pi}{4}} \int_0^{\sec\theta} r^2 dr d\theta$$
The integrand r^2 is the transformation of $$\sqrt{x^2+y^2}=r$$ multiplied by the Jacobian $$rdrd\theta$$.
You can get the bounds for the upper triangle (where t varies from t=pi/4 to t=pi/2) similarly. It's cute that sec(t) and csc(t) are straight orthogonal lines in polar coordinates.

Last edited: Mar 4, 2005
10. Mar 4, 2005

Sorry, I'm being hardheaded. :( I'm just getting these conflicting reports that it wouldn't be 'that hard' to convert to polar coordinates, and now I hear it can't be done at all. So does that mean my original method setting u=x^2 and v=y^2 is the most efficient way? Or are there alternative(s)?

11. Mar 4, 2005

### hypermorphism

Do you just mean it's nontrivial, because the transformation is pretty straightforward. :tongue2:

12. Mar 4, 2005

### dextercioby

What...?No,the 1/4 of the circle of radius 1 in the 1-st quadrant does not cover the whole are of the square [0,1]*[0,1]...

Daniel.

13. Mar 4, 2005

Thank you SO much hypermorphism! Wow, you seem to have this stuff down pat. Impressive! I'm still a bit lost as to the orientation of the triangles...are you drawing the triangle from (0,0) to (1,1) and splitting the box that way...or what orientation are you splitting it from? I'm just trying to get a better visual feel of what you are trying to convey.

So for the upper triangle I would just do the following

$$\int_{\frac{\pi}{4}}^{\frac{\pi}{2}} \int_0^{\csc\theta} r^2 dr d\theta$$

Or maybe that is a bit off. I think it'll all come together once I get a feel for the graph of the situation. Thanks again!

14. Mar 4, 2005

### hypermorphism

Of course not. That's the lower triangle. The upper triangle is from pi/4 to pi/2 and integrates csc(t). I had to let the original poster do *some* work at creating the bounds, or they won't understand the change fully. :tongue2:

15. Mar 4, 2005

### dextercioby

Which lower triangle are u talking about...?The one with a 1/4 of a circle as a curved side...?

Daniel.

16. Mar 4, 2005

### hypermorphism

Polar coordinates have a variable radius tethered to the origin that holds angle t with the positive x-axis (in reference to Cartesian coordinates). Thus the triangles are formed by the line connecting (0,0) and (sqrt(2), pi/4) in polar coordinates, or in Cartesian the line formed by (0,0) and (1,1).
Yep.

17. Mar 4, 2005

### hypermorphism

No, that would be
$$\int_0^\frac{\pi}{4} \int_0^1 r dr d\theta$$
You do realize that the graph of $$r=\sec\theta$$ is a straight line in polar coordinates ?
Note that the volume of the triangle
$$\int_0^1 \int_0^x dy dx =\frac{1}{2}$$
is easily translated to polar coordinates
$$\int_0^{\frac{\pi}{4}} \int_0^{\sec\theta} r dr d\theta = \frac{1}{2}$$

Last edited: Mar 4, 2005
18. Mar 4, 2005

### dextercioby

Yes,so what...?

Daniel.

19. Mar 4, 2005

### hypermorphism

Basically, what are you talking about ?

20. Mar 4, 2005

### dextercioby

I'm just waiting to see what trick you pull to convert the square [0,1]*[0,1] in itself by a change of variable...

Could you come up with a drawing...?

Daniel.