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Homework Help: Parabolic arch question

  1. Feb 21, 2008 #1
    1. The problem statement, all variables and given/known data

    A parabolic arch spans a stream 200 feet wide. How high
    must the arch be above the stream to give a minimum
    clearance of 40 feet over a channel in the center that is
    120 feet wide?

    2. Relevant equations

    3. The attempt at a solution

    They are asking for the k value right?
    If I place the parabola starting from x=0,y=0 then the vertex is at (100,40+n).
    When x=40,y=40. x=160,y=40 I want to find out what y is when x=100.

    So I used the equation (x-h)^2 = a(y-k)

    (x-100)^2 = a(y-(40+n)) After using x=40 ,y=40 I get n=-3600/a And now I have no idea what to do .. Am I doing this right?
  2. jcsd
  3. Feb 21, 2008 #2


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    I think so. Why not just put x=0 and y=0 into your equation and get another relation between 'a' and 'n'?
  4. Feb 21, 2008 #3
    I think you can just reason it out, no need for all those crazy equations and what have you

    Basically you're gonna have a parabola that crosses the x-axis at x=-100 and x=100

    You need at least y=40 at x=60 and -60, that ultimately what it's asking, right? And it's basically of the form Y=-Ax^2+H (I just centered it at 0 for ease)

    H is what we're looking for, and A is also unknown, however we know if y=40, then x=60, and if y=0, x=100

    I went ahead and finished this out to find A and H and checked it and it was right, see if you can follow.
  5. Feb 21, 2008 #4
    Ok I used the equation y=-ax^2+h with the values x=60,y=40 and x=100,y=0.
    Then solved the system and got a = -1/160 and h = 62.5 (the answer for the question).
    And the equation of the parabola is y=-1/160x^2+62.5

    Thanks for the help :). This is the first time i've done a question like this and I had trouble approaching it. I'm self studying these days and this shows i got a long way to go.
  6. Feb 21, 2008 #5
    I feel your pain, I been self-studying for months now. I'm proud of ya :-] Keep at it!!!
  7. Feb 22, 2008 #6


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    I performed a process somewhat like what was described in #3 and #4, but obtained a somewhat different result. The standard form of a parabola was still necessary.

    Do you know of any other interesting problems like this; using parabolas or other conic sections? Interesting and varied problems are often difficult to find. Also, I'm curious; was the question in post #1 from PreCalculus, or was it from Intermediate Algebra (I suspect it is from PreCalculus). Once in a while, I restudy College Algebra or Intermediate Algebra, and the applied situation exercises are often interesting but I just do not find enough of them.
  8. Feb 22, 2008 #7
    What result did you get symbolipoint? The book says the answer is 62.5 which i got.
    I am studying conics right now and I'll let you know if I find any more similar problems. This is from a intermediate algebra book. You are correct that most books do not have many problems like these. They are indeed interesting :)
  9. Feb 23, 2008 #8


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    I just wrote a lengthy response and the forum cut me off.

    In short, [tex] \[
    f(x) = - \frac{1}{{90}}x^2 + 40 + 71{\textstyle{1 \over 9}}

    I used an "untranslated" parabola, and then a "translated" parabola. Too difficult to rewrite all the details NOW. One used x=60, the other relied on x=100.
  10. Feb 24, 2008 #9
    Hmm I think you made a mistake somewhere. Can someone confirm the answer please? What did you get blochwave?
  11. Feb 26, 2008 #10


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    Blockwave understood the problem description. I may have obtained the "wrong" answer because I did not fully understand the problem description. Yet, he seems to have taken most of the approach that I took. Are we both misunderstanding something?
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