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Maximize Area

  1. May 15, 2009 #1
    1. The problem statement, all variables and given/known data

    Note the picture

    2. Relevant equations



    3. The attempt at a solution

    A of both trianles = xsinQ[tex]\sqrt{x^2 - x^2(sinQ})^2[/tex]

    A of rectangle = (w-2x)(xsinQ)

    Total A = (w-2x)(xsinQ) + xsinQ[tex]\sqrt{x^2 - x^2(sinQ)^2}[/tex]

    A = wxsinQ + 2x2sinQ + x2sinQcosQ

    Ax = wsinQ + 4xsinQ + 2xsinQcosQ = 0

    -wsinQ = x(4sinQ + 2sinQcosQ)

    x = -w/(4 + 2cosQ)

    AQ = wxcosQ + 2x2cosQ + x2(sinQ)2 - x2(cosQ)2 = 0

    It gets pretty messy when i plug in x, are there anny errors so far
     

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  3. May 17, 2009 #2
    anybody, can i get some help
     
  4. May 17, 2009 #3

    Mark44

    Staff: Mentor

    What's the problem you are trying to solve? The picture doesn't give me any idea of what you're trying to do.
     
  5. May 17, 2009 #4
    you must Maximize the area, note the title
     
  6. May 18, 2009 #5

    Mark44

    Staff: Mentor

    Some context would have been nice, such as that the area is the cross-sectional area of what looks to be a trough, formed from sheet metal that is W units wide.

    Your picture doesn't convey that information, which would have been helpful.

    You have a sign error in this line:
    The first '+' sign should be '-'. That makes Ax = WxsinQ - 4xsinQ + 2xsinQcosQ = sinQ[W -4x + 2xcosQ].

    The sign error also affects AQ, which I get as xWcosQ -2x2cosQ + x2cos2Q - x2sin2Q.
    About the only thing I can think of to do is to factor x out of each term in this partial.

    The critical points are where Q = 0 or when x = 0, either of which gives you an area of 0, and whatever you get from some messy equations that remain. One of these is W - 4x + 2xcosQ = 0, which you can solve for x in terms of Q (and W), or solve for Q in terms of x (and W). Whichever variable you solve for, substitute that in the other equation that comes from AQ.
     
  7. May 18, 2009 #6
    Ya, this is pretty much where i ran into problems, i pluged in my x = w/(4-2cosQ) into A sub Q

    AQ = 0 = (w2cos)/(4-2cos) - 2w2cos/(4-2cos)2 + (2w2cos2)/(4-2cos)2 - (w2sin2)/(4-2cos2)

    I would have to solve that for Q (i got lazy and didnt type them, just assume they are there), which means (tell me if im wrong), i only have to solve the numerators of the expression for 0 in terms of w

    I got Q = 0 & 2pi which would make the gutter flat, what happened
     
  8. May 18, 2009 #7

    Mark44

    Staff: Mentor

    I didn't work it through that far, so I'll assume that the work you have in post 6 is correct. You can't just "solve the numerators," as you put it, since not all of your expressions have the same denominator; the first has a denominator of 4 - 2cosQ. You'll need to multiply that term by (4 - 2cosQ)/(4 - 2cosQ), and then you can factor out 1/(4 - 2cosQ)2 from all four terms. Finally, you'll need to see what values of Q (theta) make AQ equal to 0.

    As I said before, Q = 0 and x = 0 are critical values, but they produce a trough with cross-sectional area 0, so they are not the values you want.
     
  9. May 18, 2009 #8
    ok i was able to factor out the w^{2} and the (4-2cos)^{2} and I got

    4cos - 2cos^{2} - 2cos + cos^{2} - sin^{2} = 2cos - con^{2} - sin^{2} = 0


    I got Q = pi/3, 5pi/3, 7pi/3

    how do i calc which one is the maximum, there are many possibilities
     
  10. May 18, 2009 #9

    Mark44

    Staff: Mentor

    Due to the fact that you're working with a physical object, a trough, there is a natural range of values for Q (theta). That should help you eliminate many of the possibilities. If Axx < 0 and AQQ < 0, you're at a local maximum.
     
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