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Homework Help: Optimization Problem argh

  1. Jul 12, 2007 #1
    1. The problem statement, all variables and given/known data
    The Question is:
    "A 400km racetrack is to be built with two straight sides and semicricles at the ends. Find the dimensions of the track that encloses the maximum area."

    The two long sides of the rectangle are written with >/= to 100m (each)
    The straight side of the 2 semi circles is written with >/= to 20m (each)
    (>/= means greater than or equal to, just in case)

    2. Relevant equations

    Area of semicircle = 1/2 * pr2
    Area of rectangle = lw

    3. The attempt at a solution
    Well i tried but its always the beginning setting up of optimization problems that is the killer, the rest is always easy. It is a study Q for an upcoming test.
  2. jcsd
  3. Jul 12, 2007 #2


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    Gold Member

    Find a way to write the length of the straight section in terms of the radius of the end circles. To do this, you need to use the fact that the total perimeter is 400km.
  4. Jul 13, 2007 #3


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    Science Advisor

    In the semicircle ends, r= w/2 where w is the "width" of the rectangle. Since there are two ends, you really have one circle. The area you want to maximize is [itex]\pi r^2= \pi/2 w^2[/itex]. As danago said, the circumference of the figure is 400 m so [itex]2l+ \pi/4 w^2= 400[/itex].
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