`y=x^3/2 , y=0 , x=3` Use the shell method to set up and evaluate the integral that gives the volume of the solid generated by revolving the plane region about the y-axis.

Expert Answers
marizi eNotes educator| Certified Educator

To be able to use the shell method, a rectangular strip from the bounded plane region should be parallel to the axis of revolution. By revolving multiple rectangular strip, it forms infinite numbers of these hollow pipes or representative cylinders.

 In this method, we follow the formula: `V = int_a^b ` (length * height * thickness)

or `V = int_a^b 2pi` * radius*height*thickness

where:

radius (r)= distance of the rectangular strip to the axis of revolution

height (h) = length of the rectangular strip

thickness = width  of the rectangular strip  as `dx` or `dy` .

For the bounded region, as shown on the attached image, the rectangular strip is parallel to y-axis (axis of rotation). We can let:

`r=x`

`h=f(x)` or `h=y_(above)-y_(below)`

`h= x^3/2-0 =x^3/2`

thickness `= dx`

 Boundary values of `x` from `a=0` to `b =3` .

Plug-in the values on `V = int_a^b 2pi` * radius*height*thickness, we get:

`V = int_0^3 2pi*x*(x^3/2)*dx`

Simplify: `V = int_0^3 pi*x*(x^3)dx`

Apply Law of Exponent: `x^n*x^m = x^((n+m)).`

`V = int_0^3 pi*x*(x^3)dx`

`V = int_0^3 pi(x^4)dx`

Apply basic integration property: `intc*f(x) dx = c int f(x) dx` and Power rule for integration formula : `int x^n dx =x^(n+1)/(n+1)` .

`V = int_0^3 pi(x^4)dx`

  `= pi* int_0^3 (x^4)dx`

  ` = pi*x^((4+1))/((4+1))|_0^3`

   ` =(pix^5)/5|_0^3`

Apply the definite integral formula: `int _a^b f(x) dx = F(b) - F(a)` .

`V =(pi(3)^5)/5 -(pi(0)^5)/5`

`V = (243pi)/5 -0`

`V =(243pi)/5 ` or `152.68` (approximated value)

 

Images:
This image has been Flagged as inappropriate Click to unflag
Image (1 of 1)