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Quadratic Regression Word Problems

  1. Sep 29, 2012 #1
    1. The problem statement, all variables and given/known data
    A rectangular parking lot is to be placed with one side along a building where no fence is required. If 1000 yards of wire are available for fencing the other 3 sides of the lot, what
    is the maximum area the lot may have, and what are the dimensions of the lot of a maximum area?

    2. Relevant equations



    3. The attempt at a solution
    i used my graphing calculator to figure out the equation or at least what i think is the equation -x² +1000, i think you have t subtract something because you dont need one side but im not sure what you subtract, maybe its -250 since its 1/4 of it, im not sure
     
  2. jcsd
  3. Sep 29, 2012 #2

    LCKurtz

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    ##-x^2+1000## isn't an equation, it is an expression. And it has nothing to do with the problem anyway. And how do you use a graphing calculator in the first place without knowing what to graph?? You need to set up appropriate equations for the perimeter and area and use calculus.
     
  4. Sep 29, 2012 #3
    oh no my bad i mean -x² + 1000x, i used a ti 83 and i made a table . Is the answer -636.08247422674x²+394123.71134017x-26319587.628863
     
  5. Sep 29, 2012 #4

    LCKurtz

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    No. And -x² + 1000x is still just an expression, not an equation. And the question asks for the dimensions, not a polynomial in x. Draw a picture, label the unknowns, and write down some equations, complete with = signs to get started. Put the graphics calculator away.
     
  6. Sep 29, 2012 #5

    Ray Vickson

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    What is x supposed to represent? How on Earth do you start with -x2 + 1000x and end up with -636.08247422674x²+394123.71134017x-26319587.628863? That makes no sense at all.

    Suggestion: start by drawing a picture, clearly labelling the items of interest. Assign symbols such as x, y, z, etc., to the various unknown quantities, then try to see if there are some relationships between them which allows you to decrease their number. In particular: you have 1000 yds. of wire. How do you express that fact in terms of your unknowns? What is the enclosed area as expressed in terms of your unknowns? Try to be systematic, and to stop along the way to explain what you are doing.

    RGV
     
  7. Sep 29, 2012 #6
    im so confused could someone just explain it to me very clearly, my whole life math has been my favourite subject but this is the only topic in math that i dont understand, i have a reputation to maintain, everyone in class is expecting me to figure this out and if i dont i lose major street cred
     
  8. Sep 29, 2012 #7

    LCKurtz

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    Why don't you try was has been suggested to you, either by me above or Ray Vickson another post? Show us the equations you start with and maybe we can see where you are stuck.

    Also, since this is an academic type forum we encourage the use of correct English such as I instead of i for the personal pronoun. Sorry about the "street cred".
     
  9. Sep 29, 2012 #8
    ok, forget about the x and 1000 i was way of. i made a table with 4 columns: in the first column i labeled it length, second length, third width and last area.
    Length Length Width Area
    100 100 800 8000000
    200 200 600 24000000
    300 300 400 36000000
    400 400 200 32000000
    450 450 100 20250000

    I got the maximum area to be approximately 310, 310, 380, 36518000
     
  10. Sep 29, 2012 #9

    LCKurtz

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    So you apparently aren't interested in learning how to solve the problem. And, unfortunately, you can't even calculate areas correctly. I hope none of your peers see this because it will surely hurt your street cred. :frown:
     
  11. Sep 30, 2012 #10

    vela

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    OK, so you seem to have figured out how to calculate the width of the rectangle if you're given a number for the length. Now say the length is equal to x. Can you tell us what the width is in terms of x?

    Next, as LCKurtz pointed out, you didn't calculate the area correctly. Think again about how you find the area of a rectangle. Do you see why that column of your table is wrong?
     
  12. Sep 30, 2012 #11
    oh i accidentally multiplied all of them together, ill do it again, thanks Vela

    Mod note: Removed insult
     
    Last edited by a moderator: Sep 30, 2012
  13. Sep 30, 2012 #12

    vela

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    So can you answer my first question? If x is the length of one side, what expression gives the area of the rectangle?
     
  14. Sep 30, 2012 #13
    lenth times width and whats an infraction
     
  15. Sep 30, 2012 #14
    i changed the area and it is 80000, 120000, 120000, 80000, but i took out the 450
     
  16. Sep 30, 2012 #15
    so would the equation be -2x² + 1000x and the maximum area is 125000 and the dimesions are 250,250,500, is this right
     
  17. Sep 30, 2012 #16

    Ray Vickson

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    It is impossible to answer your questions (actually, not questions, since they are not followed by a question mark "?"---but we can guess what you mean). Why impossible? Because, for the simple reason that you never tell us what x is supposed to be. Also: just writing down a formula without explanation is no good: you need to explain how you get it. (That way, if the formula is wrong we can help you figure out why it is wrong.)

    RGV
     
  18. Sep 30, 2012 #17

    vela

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    Read the forum rules, the ones you agreed to when you signed up.

    I have no idea what this means.

    That is the correct expression for the area of the rectangle. When you say equation, there has to be an equal sign somewhere.

    Your area and dimensions are incorrect because you didn't include any units. Is it 250 miles on one side? 250 inches? 250 meters? 250 nanometers? Numerically, though, they're fine, if you have the right units.
     
  19. Sep 30, 2012 #18
    i dont know what x is, could someone please explain it to me and tell me the answer so then i could work backwards please, im not cheating i will work backwadr
     
  20. Sep 30, 2012 #19

    vela

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    How can you not know what x is? What did you have in mind when you wrote down ##-2x^2+1000x##?
     
  21. Sep 30, 2012 #20
    i just put it into my graphing calculator (ti83) and it gave me an equation if you want i could tell you exactly what i did
     
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