. A field will be made in the shape of a rectangle with an area of 400 square meters. One side of the field is along a river and a fence will be built along the other three sides. A brick wall perpendicular to the river will be built to divide the field into two equal halves. the wall costs $20 per meter and the fence costs $10 per meter to build. what is the lowest possible cost to build such a field?

The river is supposed to be along the longest side so that the price of fencing the other three sides is minimum. Thus the total perimeter of the fence is b+ b+ l = 2b+l.

Total cost for fencing the other sides of the field = $ 10 × (2b + l)

The wall is supposed to be perpendicular to the river and thus the length of the wall is b meters.

Total cost for the wall is $ 20 × b

Therefore, the total price for making the field is given by

C = 10 × (2b + l) + 20 × b

⇒ C = 40b + 10l

⇒ C = [tex]\frac{16000}{l}[/tex] + 10l

To minimize the cost we differentiate the cost with respect to l and equate it to zero.

[tex]\frac{dC}{dl}[/tex] = 0 = - [tex]\frac{16000}{l^{2}}[/tex] + 10

⇒ [tex]l^{2}[/tex] = 1600

⇒ l = 40 ; [ negative sign neglected as length cannot be negative ]

⇒ b = 10

The second order derivative of C is positive giving the minimum value of the cost.

Thus the minimum cost required to make the field is given by $800.