A home improvement contractor is painting the walls and ceiling of a rectangular room. The volume of the room is 875.00 cubic feet. The cost of wall paint is $0.08 per square foot and the cost of ceiling paint is $0.14 per square foot. Find the room dimensions that result in a minimum cost for the paint.

The room dimensions for a minimum cost are: sides of 10 feet and height of 8.75 feet.

Step-by-step explanation:

We have a rectangular room with sides x and height y.

The volume of the room is 875 cubic feet, and can be expressed as:

With this equation we can define y in function of x as:

The cost of wall paint is $0.08 per square foot. We have 4 walls which have an area Aw:

The cost of ceiling paint is $0.14 per square foot. We have only one ceiling with an area:

We can express the total cost of painting as:

To calculate the minimum cost, we derive this function C and equal to zero:

The sides of the room have to be x=10 feet.

The height can be calculated as:

The room will have sides of 10 feet and a height of 8.75 feet.

x = ±sqrt(10)

step-by-step explanation:

5 - 2x² = -15

subtract 5 from each side

5-5 - 2x² = -15-5

-2x^2   = -20

divide by -2 on each side

-2x^2   /-2= -20/-2

x^2 =10

take the square root on each side

sqrt(x^2 )=±sqrt(10)

x = ±sqrt(10)

check:

5 - 2x² = -15

+sqrt(10)

5 - 2(sqrt(10))² = -15

5 -2(10) = -15

5 -20 = -15

-15 = -15

-sqrt(10)

5 - 2(-sqrt(10))² = -15

5 -2(10) = -15

5 -20 = -15

-15 = -15