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January 25, 2015

January 25, 2015

Posted by **Beth** on Thursday, November 3, 2011 at 8:38pm.

b) Find the volume of the solid obtained by rotating the region bounded by y = 4x^2, x = 1, and y = 0 about the x-axis.

c) Find the volume of the solid obtained by rotating the region bounded by y = 1/x^6, y = 0, x = 3, and x = 9, about the x-axis.

Thanks for your trouble.

- Calculus -
**Steve**, Friday, November 4, 2011 at 11:50amI'll do (a)

First, determine where the curves intersect:

6y = y^3 at the point (6√6,√6)

You can do this using discs or shells. Using discs, or washers, you have a stack of washers. The area of a disc of outer radius R and inner hole of radis r is

m(R^2 - r^2)

Since 6y > y^3 on the interval desired

The thickness of each disc is dy. So, the volume of the stack of discs is the integral

π Int(R^2 - r^2 dy)[0,√6]

= π Int(36y^2 - (y^3)^2) dy)[0,√6]

= π Int(36y^2 - y^6 dy)[0,√6]

= π (12y^3 - 1/7 y^7)[0,√6]

= π (12*6√6 - 1/7 * 216√6)

= 36π√6(2 - 6/7)

= 36*8π√6/7

Or, if you want to calculate using shells, the volume of a shell is 2πrh

r = x, h = x^(1/3) - x/6

So, the volume of a ring of shells is

2π Int(x(x^1/3 - x/6) dx )[0,6^{3/2}]

= 2π Int(x(x^1/3 - x/6) dx )[0,6^{3/2}]

= 2π Int(x^{4/3}- x^{2}/6) dx )[0,6^{3/2}]

= 2π(3/7 x^{7/3}- x^{3}/18)[0,6^{3/2}]

= 2π(6^{7/2}- 6^{9/2}/18)

= 2π*216√6(3/7 - 1/3)

= π*216√6(4/21)

= 36*8π√6/7

- Calculus -
**Beth**, Friday, November 4, 2011 at 1:21pmThank you!

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