# Surface integral over an annulus

1. Aug 31, 2010

### bobred

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

Find the area integral of the surface $$z=y^2+2xy-x^2+2$$ in polar form lying over the annulus $$\frac{3}{8}\leq x^2+y^2\leq1$$

2. Relevant equations

The equation in polar form is $$r^2\sin^2\theta+2r^2\cos\theta\sin\theta-r^2\cos^2\theta+2$$

$$\displaystyle{\int^{\pi}_{-\pi}}\int^1_{\sqrt{\frac{3}{8}}}r^2\sin^2\theta+2r^2\cos\theta\sin\theta-r^2\cos^2\theta+2\, r\, dr\, d\theta$$

3. The attempt at a solution
Hi, would just like to know if what I have done is correct.

Integrating with respect to $$r$$

$${\frac {55}{256}}\, \left( \sin \left( \theta \right) \right) ^{2}+{ \frac {55}{128}}\,\cos \left( \theta \right) \sin \left( \theta \right) -{\frac {55}{256}}\, \left( \cos \left( \theta \right) \right) ^{2}+5/8$$

and with respect to $$\theta$$, I get the area as

$$\frac{5}{4}\pi$$

Does this look ok?

2. Aug 31, 2010

### lanedance

effect, you've performed the integral
$$\int \int z(r,\theta) dr d \theta$$

i'm not really sure what that represents, you need to find an area element dA and sum up all the elements over the r, theta range

note that an area element changes with coordinate representation, for example for a flat area element
$$dA = dxdy = rdr d \theta$$

if you performed
$$\int \int z(r,\theta) dA = \int \int z(r,\theta) r dr d \theta$$

that would give you the volume of the surface above the xy plane

Last edited: Aug 31, 2010
3. Aug 31, 2010

### lanedance

4. Aug 31, 2010

### lanedance

updated post #2

5. Aug 31, 2010

### bobred

So am I right in thinking I need to workout

$$\int \int \sqrt{(\frac{\partial}{\partial r})^2+(\frac{\partial}{\partial \theta})^2+1} r dr d \theta$$

James

6. Aug 31, 2010

### Dick

Not really. That's not the form given in the reference lanedance gave. The derivatives are with respect to the wrong variables.

7. Aug 31, 2010

### hunt_mat

I compute the answer as 5\pi /8

Mat

8. Aug 31, 2010

### jackmell

That's kinda' confusing Bob. It's just the area of a surface right? If so, then first start with just the formula for the surface area of the function z=f(x,y) over some region R:

$$A=\int\int_R \sqrt{1+z_x^2+z_y^2}\;dxdy$$

See, that's nice and clean and no one would complain. You can do those derivatives right?

You end up with a very clean integral:

$$\int\int_R \sqrt{1+8y^2+8x^2}\; dxdy$$

How convenient is that! Now switch to polar coordinates with r^2=x^2+y^2 and that dxdy=rdrdt. Can you now figure the integration limits in polar coordinates?

9. Sep 1, 2010

### bobred

Got there in the end

\begin{aligned}\iint_R\sqrt{8r^2+1}\,rdrd\theta &= \int_0^{2\pi}\int_{\sqrt{3/8}}^1\sqrt{8r^2+1}\,rdrd\theta \\ &= \int_0^{2\pi}\Bigl[\tfrac1{24}(8r^2+1)^{3/2}\Bigr]_{\sqrt{3/8}}^1\,d\theta \\ &= \int_0^{2\pi}\!\!\tfrac{19}{24}\,d\theta = \tfrac{19}{12}\pi.\end{aligned}