Question

Consider the following. x = 6 sin y , 0 ≤ y ≤ π, x = 0; about y = 4

Answer 1

Answer:

12pi(8-pi), or

183.158 to third decimal place

Step-by-step explanation:

The geometry is indicated in the attached figure.

A. by integration

We will find the volume of the solid by the method of shells, i.e. we will integrate strips parallel to the axis of rotation to form many thin shells, then integrate to get the sum of all these shells.

For each shell, of thickness dy, we integrate strips of length located at y

L(x) = y(x)

and area

L(x)dy

Each strip is at a distance of (4-y) from

for which the volume of each shell equals

dV = 2*pi*(4-y)*L(x)dy = 2*pi*(4-y)*y(x) dy

The total volume of the solid can be obtained by integrating y from 0 to pi

integral( dV ) from 0 to pi

= integral (2*pi*(4-y)*y(x) dy) for y from 0 to pi

= 12*pi(-sin(y)+y*cos(y)-4*cos(y)) for y from 0 to pi

=12(8-pi)*pi

= 183.158

B. Using Pappus theorem

Pappus theorem simplifies the calculation of volume of revolution by multiplying the area of the rotating region by 2pi times the distance between the centroid and the rotation axis.

Here the area of the figure is A=2*6=12, (2 is the area under the sine curve from 0 to pi), or

A = integral (6sin(x))dx, x from 0 to pi

= 6 cos(x), x from 0 to pi

= 6(1- (-1))

= 12

Distance from centroid to axis of rotation = (4-pi/2)

Volume = 2*pi*A*(4-pi/2) = 2*pi*12*(4-pi/2)

= 12pi(8-pi)

=183.158    as before