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Math Help - Completing the square.

  1. #1
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    Completing the square.



    I tried to complete the square (I think thats what it's called) when I multiplied (-2)(15)=-30 and get the factors of -30 that add up to -12 but none of them work e.g (-15) and (2) add up to -13 instead of -12.

    I have no clue how to get it into the form that they have in the choices. Can anyone help me out?
    Last edited by mr fantastic; July 21st 2011 at 09:02 PM. Reason: Re-titled.
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  2. #2
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    Re: Help with math problem

    -2x^2 - 12x + 15

    -2(x^2 + 6x) + 15

    -2(x^2 + 6x + 9) + 15 + 18

    -2(x+3)^2 + 33
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  3. #3
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    Re: Help with math problem

    Quote Originally Posted by skeeter View Post
    -2x^2 - 12x + 15

    -2(x^2 + 6x) + 15

    -2(x^2 + 6x + 9 - 9) + 15

    -2(x^2 + 6x + 9) + 15 - (9)(-2)

    -2(x^2 + 6x + 9) + 15 + 18

    -2(x+3)^2 + 33
    Thank you. I didn't get what you did at first, then I realized the steps you did in your head automatically. And I didn't know that you could factor only part of an equation (i.e. the first two terms but not the 15)
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  4. #4
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    Re: Help with math problem

    Quote Originally Posted by Fenixx09 View Post
    Thank you. I didn't get what you did at first, then I realized the steps you did in your head automatically. And I didn't know that you could factor only part of an equation (i.e. the first two terms but not the 15)
    Kudos to you for thinking ... it's a rare activity these days.
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