# Integration by filaments or integration by strate?

Amaelle
Homework Statement:
look at the image
Relevant Equations:
cylindrical coordinates
integration by strates
integration by filaments
Greetings
While solving the following exercice, ( the method used is the integration by filaments and I have no problem doing it this way) here is the solution  My question is the following:
I want to do the integration by strate and here is my proposition is that even correct?
I would like to know if there is cases when switching between the two methods of integration is not possible?

• Delta2

Staff Emeritus
Homework Helper
I've never heard of "integration by strate." Could you explain what this means?

• • Delta2 and Amaelle
Amaelle
Certainly look at the image it means doing the integration by rings and then integrating by hight

Staff Emeritus
Homework Helper
Oh, okay. In the US, I think that technique is referred to as the disc method.

• Delta2
Staff Emeritus
Homework Helper
You should be able to use that method, but you're going to have to break the integral up into two integrals, one from ##x=-2## to ##x=-\sqrt{2}## and one from ##x=-\sqrt{2}## to ##x=-1##.

• • Delta2 and BvU
Amaelle
thank you very much , indeed I used your insights to solve the question under the disk method but the only way I could find out was to and then substructing the sphere from the paraboloid
I would be very grateful if you could elaborate more about the setting of your method and why you need to split the integration this way?
thank you!

• Delta2
Staff Emeritus
Homework Helper
To keep it simple, I'm going to consider volume integrals but the same ideas apply to your problem.

When you do the integrals over ##r## and ##\theta##, you end up with an integral of the form
$$\int_{x_1}^{x_2} \pi r^2 \,dx$$ where we can identify ##\pi r^2## as the area of the disk and ##dV = \pi r^2\,dx## as the volume of the infinitesimal slice. In this particular problem, from ##x=-2## to ##x=-\sqrt{2}##, the volume is given by an integral of that form. However, from ##x=-\sqrt{2}## to ##x=-1##, you have a disk with a hole in the middle with area ##\pi(r_{\rm outer}^2-r_{\rm inner}^2)##. Hence, the volume of that portion is given by
$$\int_{x_1}^{x_2} \pi (r_{\rm outer}^2-r_{\rm inner}^2) \,dx.$$ Because you have two different integrands, you need to do the integrals for each range separately.

Note that you can manipulate your integrals similarly:
\begin{align*}
I &= \int_{-2}^{-1} \int_0^{2\pi} \int_0^{\sqrt{2-x^2}} xr\,dr\,d\theta\,dx
-\int_{-\sqrt 2}^{-1} \int_0^{2\pi} \int_0^{\sqrt{x+2}} xr\,dr\,d\theta\,dx \\
&= \int_{-2}^{-\sqrt{2}} \int_0^{2\pi} \int_0^{\sqrt{2-x^2}} xr\,dr\,d\theta\,dx
+\int_{-\sqrt{2}}^{-1} \int_0^{2\pi} \int_0^{\sqrt{2-x^2}} xr\,dr\,d\theta\,dx
-\int_{-\sqrt 2}^{-1} \int_0^{2\pi} \int_0^{\sqrt{x+2}} xr\,dr\,d\theta\,dx \\
&= \int_{-2}^{-\sqrt{2}} \int_0^{2\pi} \int_0^{\sqrt{2-x^2}} xr\,dr\,d\theta\,dx
+\int_{-\sqrt{2}}^{-1} \int_0^{2\pi} \int_{\sqrt{x+2}}^{\sqrt{2-x^2}} xr\,dr\,d\theta\,dx
\end{align*} The limits on the ##r## integral are different for ##x=-2## to ##x=-\sqrt{2}## and for ##x=-\sqrt{2}## to ##x=-1##.

• Amaelle
Amaelle
thanks a million!