How do you find the volume of the solid obtained by rotating the region bounded by the curves #y=sqrtx# and #y=x/3# rotated around the #x=-1#?

Answer 1

See the explanation section, below.

Here is a picture of the region (in blue) and the line #x=-1#.

It shows a representative slice of thickness #dx#. The slice is taken parallel to the axis of rotation, so when we rotate we'll get cylindrical shell.
The volume of the shell is #2pirhdx#. The radius, #r#, is shown by the dotted red horizontal line. Its length is #x+1#.
The height of the shell is the greater #y# value minus the lesser #y# value. That is: #h = sqrtx-x/3#

We also need the point of intersection where #x/3=sqrt3#. Which happens at #0# and at #9#

The integral we need to evaluate is:

#int_0^9 2 pi (x+1)(sqrtx-x/3) dx#

Expand the integrand to get 4 terms in powers of #x# which is straightforward to evaluate.

#2 pi int_0^9 (x+1)(sqrtx-x/3) dx = 207/5pi#

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Answer 2

To find the volume of the solid obtained by rotating the region bounded by the curves y=xy=\sqrt{x} and y=x3y=\frac{x}{3} around the line x=1x=-1, you can use the method of cylindrical shells.

The limits of integration will be determined by the points where the two curves intersect. Setting x=x3\sqrt{x} = \frac{x}{3}, you can solve for xx to find the intersection point.

x=x3\sqrt{x} = \frac{x}{3}
x=x29x = \frac{x^2}{9}
9x=x29x = x^2
x29x=0x^2 - 9x = 0
x(x9)=0x(x - 9) = 0

x=0x = 0 or x=9x = 9

So, the limits of integration will be x=0x = 0 to x=9x = 9.

The radius rr of the cylindrical shell at xx is x+1x + 1, because the axis of rotation is x=1x = -1. The height hh of the cylindrical shell at xx is the difference between the yy-values of the upper and lower curves, which is xx3\sqrt{x} - \frac{x}{3}.

The volume VV of each cylindrical shell is given by V=2πrhV = 2\pi rh.

Therefore, the total volume is given by the integral:

V=092π(x+1)(xx3)dxV = \int_{0}^{9} 2\pi(x + 1)\left(\sqrt{x} - \frac{x}{3}\right) dx

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

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