# Optimization question about minimizing cost

• Jul 18th 2011, 02:57 AM
sara213
a new cottage is built across the river and 300m downstream from the nearest telephone relay station. the river is 120m wide. in order to wire the cottage for phone service, wire will be laid across the river under water and along the edge of the river above ground. the cost to lay wire under water is \$15 per m and cost to lay wire above ground is \$10 per m. How much wire should be laid underwater to minimize the cost?

Here is what I have:

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http://i812.photobucket.com/albums/z...equestion3.jpg

in the end I get x=3 but I don't think that is right.

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• Jul 18th 2011, 03:27 AM
TKHunny
Re: Optimization question about minimizing cost
This should be mathematics. Your opinion is not particularly important. Prove it! Check you work.

After "=0", how did you manage "=10"? Very bad. 0/10 = 0

Thank you for displaying your work.
• Jul 18th 2011, 04:18 AM
sara213
Re: Optimization question about minimizing cost
Your welcome and it should be 0/10? why not 0+10 for this reason i put 10. :S
• Jul 18th 2011, 09:04 AM
TKHunny
Re: Optimization question about minimizing cost
Solve for x:

10*x = 0

10 + x = 0

You must see this error or you need to take a week off and work on liberal arts, or something, then come back to it.