Question

A solid is formed by adjoining two hemispheres to the ends of a right circular cylinder. The total volume of the solid is 10 cubic centimeters. Find the radius of the cylinder that produces the minimum surface area. (Round your answer to three decimal places.)

Answer 1

1.619

Explanation:

We have the formulas for the volume of the sphere and the cylinder:

Vs = (4/3) * pi * (r ^ 3)

Vc = pi * h * (r ^ 2)

Thus:

(4/3) * pi * (r ^ 3) + pi * h * (r ^ 2) = 10

Now, the formula for the surface area of the cylinder and sphere are:

As = 4 * pi * (r ^ 2)

Ac = 2 * pi * r * h + 2 * pi * r ^ 2

Using this equation for the total surface area of the solid, we can see that as "h" increases, so will the surface area. Therefore, the smallest surface area will occur at h = 0.

Thus:

(4/3) * pi * (r ^ 3) + pi * 0 * (r ^ 2) = 10

(4/3) * pi * (r ^ 3) = 10

r ^ 3 = 4 * 10 / (3 * 3.14)

r ^ 3 = 4.24

r = 1.619

so the radius is 1.619