Question

A power line is to be constructed from a power station at point A to an island at point C, which is 1 mi directly out in the water from a point B on the shore. Point B is 4 mi down shore from the power station at A. It costs $5000 per mile to lay the power line under water and $3000 per mile to lay the line underground. At what point S down shores from A between the cities?

Answer 1

Given :

length offshore = CS=√(1+X^2)

Cable charged = 5000√(1+X^2)

onshore length = 4-X

laying cost = 3000(4-X)

total cost:

C=5000√(1+X^2) +3000(4-X)

DC/DX

= [5000*(0.5)*2X/{√(1+X^2)}]-3000=0... for optimum

5000X=3000√(1+X^2)

25X^2=3+3X^2

22X^2=3

X=√(3/22)

= 0.3693 miles

So, it would be laid offshore to S in a manner that

BS=X=0.3693 miles

Onshore=4-0.3693

=3.6307 miles