To find the volume of a solid by revolving the graph of y =4-x^2 about the x-axis, we consider the bounded region in between the graph and the x-axis. To evaluate this, we apply** Disk method** by using a* rectangular strip perpendicular to the axis of rotation*. As shown on the attached image, we consider a** vertical rectangular strip with a thickness =dx**.

We follow the formula for the Disk Method in a form of: `V = int_a^b pir^2 dx` or `V = pi int_a^b r^2 dx`

where *r is the length of the rectangular strip*.

In this problem, we let the *length of the rectangular strip*`=y_(above)-y_(below)` .

Then` r = (4-x^2) - 0 = 4-x^2`

Boundary values of x: `a= -2` to `b=2` .

Plug-in the values on the formula `V = pi int_a^b r^2 dx` , we get:

`V =pi int_(-2)^2 (4-x^2)^2 dx`

Expand using FOIL method:`(4-x^2)^2 = (4-x^2)(4-x^2)= 16-8x^2+x^4` .

The integral becomes:

`V =pi int_(-2)^2 (16-8x^2+x^4) dx`

Apply basic integration property:`int (u+-v+-w)dx = int (u)dx+-int (v)dx+-int(w)dx` to be able to integrate them separately using Power rule for integration: `int x^n dx = x^(n+1)/(n+1)` .

`V = pi[ int_(-2)^2(16) dx -int_(-2)^2(8x^2)dx+int_(-2)^2(x^4)dx]`

`V = pi[16x-(8x^3)/3+x^5/5]|_(-2)^2`

Apply definite integration formula:` int_a^b f(y) dy= F(b)-F(a)` .

`V = pi[16(2)-(8(2)^3)/3+(2)^5/5]-pi[16(-2)-(8(-2)^3)/3+(-2)^5/5]`

`V =pi[32-64/3+32/5]-pi[-32-(-64)/3+(-32)/5]`

`V =pi[32-64/3+32/5]-pi[-32+64/3-32/5]`

`V=(256pi)/15 -(-256pi)/15`

`V=(256pi)/15 +(256pi)/15`

`V=(512pi)/15 ` or `107.23` (approximated value)

## See eNotes Ad-Free

Start your **48-hour free trial** to get access to more than 30,000 additional guides and more than 350,000 Homework Help questions answered by our experts.

Already a member? Log in here.