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Homework Help: Optimization problem - minimize cost

  1. Apr 2, 2009 #1
    Optimization problem -- minimize cost

    1. The problem statement, all variables and given/known data

    A motor is floating on a buoy in a large pool with a perfectly flat shoreline. The motor is 3ft from the edge of the pool. A circuit to control operation of the motor is located 100ft from the closest point of the motor to the edge of the pool. Cable must be installed to control the motor. It costs 2.50/ft to run cable underwater, and 1.50/ft to run it on land. Find the proportions (amount of cable on land and amount on water) to minimize cost.

    2. Relevant equations
    Cost= (2.50w)+(1.50L) trying to minimize this function.
    where w= underwater cable length
    L= land cable length

    w^2 = (100-L)^2 + 3^2 This is the "top" triangle that is formed. 3 is a fixed length, one side of the triangle. 10-L is the other side, and w is the hypotenuse.

    3. The attempt at a solution

    C=2.50[(100-L)^2+9]^.5 + 1.50L

    Use chain rule to find C': (5/4)[(100-L)^2+9]^-.5(2L-200) + 3/2
    Set equal to zero and solve for L....right
    Rewriting this gives a rational function (because of the -1/2 power) plus a constant.

    Is this correct thus far? I tried solving this by rewriting as a rational function. I know that I only need to concern myself with the numerator of rational functions to make the whole thing equal to zero, but I have that constant out there. I tried finding a common denominator, but then the only way I can make the function equal to zero is by setting length of land cable as zero.
  2. jcsd
  3. Apr 3, 2009 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Re: Optimization problem -- minimize cost

    Yes, looks good so far.

    Can you get the square-root term so it's on one side of the equation, all by itself? Then you'd just need to square everything to get rid of the radical.
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