Associated Topics || Dr. Math Home || Search Dr. Math

### Least Cost for Laying a Pipeline

```
Date: 06/21/99 at 00:04:03
From: sandi
Subject: Trig and Calculus

I am halfway through working on a problem and am hoping you can point
me in the right direction where I am stuck.

An oil platform is at point O, at sea, 10 km from the nearest point P
on a stretch of straight coastline. An oil refinery is at point R, 16
km along the coast from point P. It is necessary to lay a pipeline
from the platform to the refinery, consisting of straight-line
sections. Laying pipeline underwater is 4/3 times as expensive as
laying pipeline on land. This ratio is the relative cost.

1\
1 \
1  \
10 1   \
1    \
1     \
1      \
1_______\______
P        X     R
<-----16km----->

If the pipeline reaches the coast at point X, distance x km from P in
the direction of R, find an expression for the cost C (in dollars) of
laying the pipeline in terms of x, given that laying pipeline on land
has a fixed cost of A dollars per kilometer.

I've worked out this expression to be:

C(x) = (4A/3)[sqrt(100 + x^2)] + A(16 - x)

Find the least expensive route for laying the pipeline. I think that
this is when x = 11.34?

This is what I'm stuck on:

Compare the cost of the solution you have found with the cost of a
pipeline directly from O to R and with a pipeline from O to P to R.

Also, if the cost of the underwater pipeline changes in relation to
the cost of the coastal pipeline, explore how the least expensive
route changes. In particular, see if there is a value of k (the
relative cost) at which the direct OR route becomes the cheapest
solution, or one for which the route from O to P to R is the least
expensive.

I am hoping that you can please give me some hints about how to go
about solving this. I am fine with theory, but with an application
problem such as this one, I don't know where to start. If you know of
any similar problems that might be able to help also, I'd appreciate
if you could let me know. Any help you can give would be very much
appreciated.

Hoping to hear from you soon,
Sandi
```

```
Date: 06/21/99 at 07:10:51
From: Doctor Floor
Subject: Re: Trig and Calculus

Dear Sandi,

Thanks for your question!

Let us consider the general case, where at sea, the costs are p (p>0)
times as much as on land.

Your function C(x) then becomes:

C(x) = pA sqrt(100+x^2) + A(16 - x)

Of course we should only consider x in the interval [0,10].

To find possible minimum costs we have to use the derivative C'(x).
This is given by:

C'(x) = pA [2x / 2sqrt(100+x^2)] - A
= pA [x/sqrt(100+x^2)] - A

(To find the derivative of sqrt(100+x^2) I used the chain-rule.)

A possible minimum should occur when C'(x) = 0. If we don't find a
solution x in [0,10], then we should check which of x = 0 or x = 10 is
the least costly.

C'(x) = 0
pA[x/sqrt(100+x^2)] - A = 0
px/sqrt(100+x^2) = 1
px = sqrt(100+x^2)
p^2x^2 = 100 + x^2
(p^2 - 1)x^2 = 100
x^2 = 100/(p^2 - 1)

This gives you possibly one solution for x in [0,10].

When p = 4/3, as it was in your first example, then you find that
x > 10, and you just have to check whether x = 0 or x = 10 gives the
desired result.

When p = 5, for example, you find that x is approx. 2.04. Then you
should still check whether this gives less costs then x = 0 or x = 10,
because from C'(x) = 0 we might find a maximum as well.

If you need more help, just write us back.

All the best,

- Doctor Floor, The Math Forum
http://mathforum.org/dr.math/
```
Associated Topics:
College Calculus

Search the Dr. Math Library:

 Find items containing (put spaces between keywords):   Click only once for faster results: [ Choose "whole words" when searching for a word like age.] all keywords, in any order at least one, that exact phrase parts of words whole words

Submit your own question to Dr. Math
Math Forum Home || Math Library || Quick Reference || Math Forum Search