# Double Integrals: cartesian -> polar and solve

1. Dec 1, 2009

### GreenLantern

Double Integrals: cartesian --> polar and solve

here is everything:

#19: I am stuck...This is to be solved using cylindrical polar coordinates and a double integral. I understand simpler ones such as find the volume of the solid under the cone z= sqrt(x^2 + Y^2) and above the disk (x^2 + y^2 ) <= 4 but i guess what is throwing me on this one is the sphere and that it is asking for area above the cone...

#13: What did i do wrong? My answer is similar to the book but not quite...

Thanks everyone!

-GL

Last edited: Dec 1, 2009
2. Dec 1, 2009

### babyEigenshep

Re: Double Integrals: cartesian --> polar and solve

19:
You have very nearly solved this one. You have found $z \geq r$ and $z \leq \sqrt{1-r^2}$, so all you need to do is find the minimum and maximum values for $r$ and you will have the triple integral
$$\int_{\min(r)}^{\max{r}}\int_r^{\sqrt{1-r^2}}\int_0^{2\pi}rd\theta dz dr$$

13:
You forgot to apply the restriction on R that says $y \leq x$

3. Dec 1, 2009

### GreenLantern

Re: Double Integrals: cartesian --> polar and solve

ughhhh okay and how exactly do i find R min/max? i hardly know how i got to where i did to be honest...

im trying to do it by visual inspection which is how my prof. said to do other ones but my mind is turning to mush thinking about it.I'd say that r max would be 1? due to part of the sphere is included and its radius is 1. Is this correct?
and is the minimum r=0 because the point of the cone?

thanks so much for replying to me though!

4. Dec 1, 2009

### babyEigenshep

Re: Double Integrals: cartesian --> polar and solve

Actually, looking at your work again, you already found $r\leq \sqrt{1/2}$. Since this is the only restriction on r, the minimum will be 0.

5. Dec 1, 2009

### GreenLantern

Re: Double Integrals: cartesian --> polar and solve

okay. and then why are we not integrating anything other than r?

6. Dec 1, 2009

### GreenLantern

Re: Double Integrals: cartesian --> polar and solve

WOWWWW
okay so totally disregard me as an asshat because earlier i was totally thinking about doing the integral of $$(r)[ \sqrt{1-r^{2}}-r]$$

with R between 0 and $$\sqrt{1/2}$$
and ø between 0 and 2π

as it would make sence to take the volume of the sphere over the cone and subtract the volume of the area under the cone....WOW i suck :(

sorry for wasting space :) and thanks for the crucial help!

-Ben

7. Dec 1, 2009

Re: Double Integrals: cartesian --> polar and solve

Do you need help with #13 still?

8. Dec 13, 2009

### GreenLantern

Re: Double Integrals: cartesian --> polar and solve

no sir i figured it out.

Thanks!