Rotating an ellipse with integrals

[wet oyster]

I am confused with this question. I know that I have to use the formula for a cylinder as each slice will be a cylinder but I am unsure how to find the radius to use for each slice

[pulsar halo]

Isn't there a formula for rotation around x-axis?

[wet oyster]

sorry wdym?

[pulsar halo]

A formula where you calc. the volume of a function that was rotated around the x axis

[amber glade]

The radius is right there on the page.

[wet oyster]

That's the answer key. I am trying to figure out the understanding behind it

[pulsar halo]

$V = \pi\int_{a}^b y^2 dx$

[bronze nova]

adonhs

[amber glade]

If you're rotating an elipses around the x axis, you want to find the function (positive) y = ... . That gives you the radius.

So all they did was solve for y

[wet oyster]

Oh wait that makes so much more sense