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Differentiation problem. - HSN forum

# Differentiation problem.

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### #1louisa567

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Posted 08 October 2006 - 11:43 AM

Our teacher has asked us to do Q4 and 5 on differentiation pg 115, Ex 6R.

4. A ship has a 200 Km journey to make at a constant speed. At x km/h the cost of the journey will be

(x squared + 4000/x) per hour.

a. Find an expression for the time taken for the journey
the total cost of the trip.

Im completely lost with part a.

b. Find the speed that minimises the cost of the journey and calculate this minimum cost.

I got as far to find x = 12.6. Is this right?

However when I do the table I get a max value rather than a minimum value.

Ive got the same problem with Q5 a and b. Any ideas?

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