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Matrix of a Parabola? (It's my first day)

  1. Aug 30, 2005 #1
    Hi guys, I started linear algebra just yesturday. I've read the first section of the first chapter and now I'm trying to do some problems from this section and I'm stuck on #7. I scanned the page from my math book (here's a link: http://img392.imageshack.us/img392/8476/lach11n72fb.jpg). After reading this section of my book, I'm pretty sure I understand what an "augmented matrix" is. I understand what "consistent" means, and also understand the basic row operations.

    In an attempt to figure out what the question is asking, I wrote down their augmented matrix in equation form, and I wrote the following.

    y1 = ax1^2 + bx1 + c
    y2 = ax2^2 + bx2 + c
    y3 = ax3^2 + bx3 + c

    Even if this isn't in the right direction to solving the problem, did I do this correctly?

    I'm confused as to what exactly the question is asking, and what to "show". I've always been bad at the questions that ask to "show" something is true or false. Please help!
     
  2. jcsd
  3. Aug 31, 2005 #2

    HallsofIvy

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    Looks like you almost have it. Yes, those equations are the ones corresponding to the given augmented matrix. Now, do you see that those are precisely the equations stating that (x1,y1), (x2, y2), (x3,y3) are on the parabola y= ax2+ bx+ c?
     
  4. Aug 31, 2005 #3
    HallsofIvy, I do see and understand that, although I'm having trouble understanding why or how that answers the question in the second phrase. Could you explain to me what the second phrase is asking me to do? Sorry for all the questions. This whole thing is totally new to me.
     
  5. Jan 15, 2009 #4
    Bump.

    I've run into the same problem from the same book. I understand that the stated equations are for each x,y point, however I'm not sure how to proceed!
     
  6. Jan 15, 2009 #5

    HallsofIvy

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    What "second question in the secnd phrase" are you talking about?
     
  7. Jan 15, 2009 #6
    when the question asks to show that (a,b,c) satisfies the system of equations, it's asking you to show that it satisfies _each_ of the equations in your system of equations. You have three equations. Each of the equations has 3 unkowns (x,y,z). One way to proceed is to show that if you substitute (a,b,c) for (x,y,z) in any of the equations, then you have a solution to that equation. To do this you must use the fact that the points (x1,y1),(x2,y2), and (x3,y3) each lie on the parabola. Also, you must use the fact that the equation of the parabola is given in the statement of the problem. I hope I haven't just confused you more. In any case, soon you will be able to answer a similar problem (if you can't already): Given three distinct points in the x-y plane, find the equation of a parabola that passes through each of them simultaneously. Perhaps when you (or if you could go ahead and) solve this related problem, #7 may seem much more simple.
     
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