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  • Mar 28, 2009, 04:40 PM
    grace14
    Quadratic equation
    I do not know how to do this problem and would really appreciate anyone who might explain it to me.
    x(2x-1)=45 it's a quadratic equation.. trying to find the diminsions of a shape.
    Grace
  • Mar 28, 2009, 04:48 PM
    galactus
    Expand it out and subtract 45 from both sides.



    It factors, so let's do that.

    What two numbers when multiplied equal -90 and when added equal -1?

    It is -90 because when we multiply the coefficient of x^2, 2, and the constant -45 we get -90. And the coefficient of x is -1.

    Let's see... what could they be?

    How about -10 and 9.



    Group:



    Factor out something common from each group:



    See? What's in the parentheses is the same. That is imperative or it won't work.



    There ya' go. See how to do them now?

    Solving for x: 2x+9=0, x=-9/2

    x-5=0, x=5

    The solutions are x=-9/2 and 5.

    It crosses the x-axis at -9/2 and 5.

    If you have to find the vertex, use

    Plug that back into the original quadratic to find the corresponding y coordinate.

    But what do you mean 'the dimensions of a shape'?

    It's a parabola if you graph it.
  • Mar 28, 2009, 04:51 PM
    grace14
    Thanks! What I meant by shape was, the problem asked me to find the dimensions of a shape, and I figured out the formula for it, I just didn't know how to slove the formula.
  • Mar 28, 2009, 04:52 PM
    galactus
    Oh, OK. I hope that helped. You could also just use the quadratic formula. That may be easier, but I like to factor if possible.
  • Mar 28, 2009, 04:54 PM
    grace14
    Yes it really did help. Thanks so much!

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