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Math Help - parabola

  1. #1
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    Exclamation parabola

    i have a HUGE math exam tomorrow and i still don't know how to do completing the square..if you have something like y=x^2 + 6x + 5 and you NEED to you use completing the square to solve it, how do you do it?

    please show it step by step

    this is what i have so far, but after this i'm a bit stuck
    y=x^2 + 6x + 5
    -5 + 9= x^2 + 9
    4= x^2 + 9

    now what do i do?

    THANK YOU FOR THE HELP!
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by ncbabe View Post
    i have a HUGE math exam tomorrow and i still don't know how to do completing the square..if you have something like y=x^2 + 6x + 5 and you NEED to you use completing the square to solve it, how do you do it?

    please show it step by step

    this is what i have so far, but after this i'm a bit stuck
    y=x^2 + 6x + 5
    -5 + 9= x^2 + 9
    4= x^2 + 9

    now what do i do?

    THANK YOU FOR THE HELP!
    y=x^2 + 6x + 5

    what do you need to add to x^2+6x to make it a perfect square?

    Well if (x+a)^2=x^2+6x +c for some c then 2a=6, and c=a^2, that is
    a=3 and c=9.

    So:

    y = (x+3)^2 - 9 +5 = (x+3)^2 - 4

    (multiply this out to check that it is the same thing that you started with.

    So now if you want to solve:

    y=x^2 + 6x + 5 = 0

    we write:

    (x+3)^2 - 4 = 0

    or:

    (x+3)^2 = 4

    so taking square roots:

    x+3 = +/- 2

    x = -5 or -1.

    Which we can check by substituting back into: x^2 + 6x + 5

    with x=-5 we have: 25 - 30 +5 =0

    and with x=-1 we have: 1 -6 +5 =0

    RonL
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  3. #3
    is up to his old tricks again! Jhevon's Avatar
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