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Math Help - pre-calc stress

  1. #1
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    pre-calc stress

    I've been staring at the following problems for at least half an hour. Would anyone help me?

    2x^2+8x=0
    -9x^2+12x-4=0
    4x^2-4x+2=0
    3x^2+4x-1=0
    x^3+2x^2-4x-8=0
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  2. #2
    Super Member 11rdc11's Avatar
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    Do you know how to use the quad formula?


    \frac{-b_-^+ \sqrt{b^2-4ac}}{2a}
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  3. #3
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    I've tried, but I keep getting answers I'm sure are way off from being right.
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  4. #4
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    What kind of answers do you keep getting?
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  5. #5
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    for the first one i got .396 and 8.48
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  6. #6
    Super Member 11rdc11's Avatar
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    I'm going to do one for you for an example

    -9x^2+12x-4=0

    Ax^2+Bx+C=0


    \frac{-12_-^+ \sqrt{144-4(-9)(-4)}}{-18}

    \frac{-12_+^- 0}{-18}

    \frac{1}{3}
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  7. #7
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    Thank you so much. I think I have an idea of how to do the rest.
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  8. #8
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    Ok, I got stuck on x^3+2x^2-4x-8=0
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  9. #9
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    Quote Originally Posted by 11rdc11 View Post
    I'm going to do one for you for an example

    -9x^2+12x-4=0

    Ax^2+Bx+C=0


    \frac{-12_-^+ \sqrt{144-4(-9)(-4)}}{-18}

    \frac{-12_+^- 0}{-18}

    \frac{1}{3}
    Yo

    Next time, you can use \pm (\pm) and \mp (\mp)
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  10. #10
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    Quote Originally Posted by nina9 View Post
    Ok, I got stuck on x^3+2x^2-4x-8=0
    Try x - 2 as a factor. x^3 + 2x^2 - 4x - 8 = (x - 2)(x^2 + 4x + 4) using long or synthetic division.
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  11. #11
    Super Member Showcase_22's Avatar
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    For 2x^2+8x=0, both the terms have x in them so this can be factorised out:

    x(2x+8)=0


    I was going to do the rest of them by finding a factor, but they all appear to be decimals or surds apart from the last one. I'll show you the method so you can use it on other questions:

    f(x)=x^3+2x^2-4x-8

    f(2)=2^3+2^3-8-8
    f(2)=8+8-8-8
    f(2)=0

    Since f(2)=0, f(x) has a factor x-2 ie. when x=2 f(x)=0.

    This can then be factorised using this (see icemanfan's post).

    A graphical method can help:



    If in doubt, draw the graph and see if that helps.
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