# B How do you find the volume?

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1. Mar 30, 2016

### EverythingIsACircle

Find the volume of the solid generated by revolving the region bounded by y = 4x-x^2 and y = 2x about the y-axis. About the line x = 2.

Since this is rotated about the y-axis, I know I have to manipulate the equation so I can get to x = something. The problem is I cannot change the equation y = 4x-x^2.

Questions:
How would you find the volume that is rotated about the y-axis while the equations are y = form instead of x =? If it is not possible, then what do I have to do in this case?
How would you find the volume if the axis of revolution is not y = 0?

2. Mar 30, 2016

### andrewkirk

You need to express x in terms of y. Use the completing the square technique.
Change variables. Replace $x$ by $x'=x-2$.

3. Apr 2, 2016

### HallsofIvy

Staff Emeritus
You can use the "cylinder method" rather that the "disk method" to integrate with respect to x. Imagine a line extending upward from each value of x. That line cuts the "region bounded by $y = 4x-x^2$ and y = 2x", for x between 0 and 2, in a line segment of length $(4x- x^2)- 2x= 2x-x^2$. Rotating around the y-axis that forms a cylinder of radius x and height $2x- x^2$ so area $2\pi x(2x- x^2)= 2\pi (2x^2- x^3)$. Taking the "thickness" of each cylinder to be "dx", the whole volume is given by $2\pi \int_0^2 2x^2- x^3 dx$.