Question

. An oil refnery is located on the north bank of a straight river that is 2 km wide. A pipeline is to be constructed from the refnery to storage tanks located on the south bank of the river 6 km east of the refnery. The cost of laying pipe is $400,000ykm over land to a point P on the north bank and $800,000ykm under the river to the tanks. To minimize the cost of the pipeline, where should P be located

Answer 1

Answer:

The point P should be located at 0,51 Km from point B, to minimize costs

Step-by-step explanation:

The oil refinery ( point A ) the opposite point ( B) in the south bank, and the storage tank (point C) these three points shape a right triangle. Let´s call x the distance between point A and point P ( the arriving point of the pipeline in the south bank ) then:

Cost = cost of pipeline in-ground* ( 6 - x ) + cost of pipeline in water* L

where L is the hypothenuse of the right triangle  ( A , B , P )

Cost ($) = 400000 * ( 6 - x ) + 800000 * √ 2² + x²

C (x) = 400000* ( 6 - x ) + 800000* √4 + x²

C(x) = 2400000 - 400000*x  + 800000* √4 + x²

Tacking derivatives on both sides of the equation

C´(x) = - 400000 + 800000* ( 2x)/√4 + x²

C´(x) = 0         - 400000 + 1600000*x / √4 + x²  = 0

- 400000* (√4 + x² )  + 1600000*x  = 0

- (√4 + x² ) + 4* x = 0

- (√4 + x² )  = - 4*x

Squaring both sides

4 + x²  = 16*x²

15*x² = 4

x² = 4/ 15

x = ± √0,27

x = ± 0,51

We should dismiss the negative solution

x = 0,51 Km

The point P should be located at 0,51 Km from point B