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Thread: Quadratic Equations

  1. #1
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    Quadratic Equations

    Am i doing this right?

    (5x+10)^2 = 40
    (5x+10)(5x+10)= 40
    25x^2 + 100x + 100 = 40
    25x^2 + 100x + 60 = 0

    once i get that last equation i do this

    (-100 (+/-) sq(10000-6000))/50

    then i simply this far

    (-80 sq(10))50 and (-120 sq(10))/50

    but the multiple choice answers dont look like what i got :/
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  2. #2
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    Re: Quadratic Equations

    $\displaystyle (5x+10)^2 = 40$

    $\displaystyle 5x+10 = \pm \sqrt{40}$

    $\displaystyle 5x+10 = \pm 2\sqrt{10}$

    $\displaystyle 5x = -10 \pm 2\sqrt{10}$

    $\displaystyle x = -2 \pm \frac{2\sqrt{10}}{5}$
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  3. #3
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    Re: Quadratic Equations

    Quote Originally Posted by DeltaHax View Post
    Am i doing this right?

    (5x+10)^2 = 40
    (5x+10)(5x+10)= 40
    25x^2 + 100x + 100 = 40
    25x^2 + 100x + 60 = 0

    once i get that last equation i do this

    (-100 (+/-) sq(10000-6000))/50

    then i simply this far

    (-80 sq(10))50 and (-120 sq(10))/50

    but the multiple choice answers dont look like what i got :/
    You are correct up to $\displaystyle \displaystyle \begin{align*} \frac{-100 \pm \sqrt{10 \, 000 - 6000}}{50} \end{align*}$, which then simplifies to

    $\displaystyle \displaystyle \begin{align*} \frac{-100 \pm \sqrt{4000}}{50} &= \frac{-100 \pm 20\sqrt{10}}{50} \\ &= \frac{-10 \pm 2\sqrt{10}}{5} \end{align*}$

    But Skeeter's method is better.
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