Good morning everyone pls help me on this question:]

L = 4 yard, W = 20 yard

Step-by-step explanation:

Area of backyard, A = 80 sq yard

Let the length of the fence is L and the width is W.

The cost of one side is $ 9 per yard and the cost of three sides is $ 1 per yard.

A = L x W = 80

W = 80 / L     .... (1)

Total cost, C = L x 9 + L x 1 + 2 W x 1

C = 10 L + 2 W

C = 10 L + 2 (80 / L)    from equation (1)

For maxima and minima differentiate C with respect to L and put it equal to zero.

dC/dL = 0

L² = 16

L = 4 yard

from equation (1)

W = 80 / 4 = 20 yard

SO, the cost is minimum is the length is 4 yard and the width is 20 yard.