Question

A cylindrical can is to be constructed for 80 cents. The cost of the material used for the top and the bottom is 3 cents per square inch, and the cost of the material used for the curved side is 2 cents per square inch. Express the volume of the can as a function of its radius. (Recall the volume of a cylinder is V

Answer 1

V = r(40 - 3πr²)/2

Explanation:

We know the area of a cylinder, A = 2πr² + 2πrh where r = radius and h = height.

Now, 2πr² = area of top and bottom and 2πrh = curved surface area

Since the cost of material for the top and bottom is 3 cents per square inch, the total cost of the materials for top and bottom = cost of material × area = 3 × 2πr² = 6πr²

Also, the cost of material for the curved is 2 cents per square inch, the total cost of the materials for the curved side = cost of material × area = 2 × 2πrh = 4πrh

The total cost of material = 6πr² + 4πrh

Since the total cost of material = 80 cents,

6πr² + 4πrh = 80

3πr² + 2πrh = 40 (1)

Now, the volume of a cylinder, V = πr²h

From (1), making h subject of the formula, we have

3πr² + 2πrh = 40

2πrh = 40 - 3πr²

h = (40 - 3πr²)/2πr

Substituting h into V, we have

V = πr²h

V = πr² (40 - 3πr²)/2πr

V = r(40 - 3πr²)/2