# Quadratic Equations

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• Dec 11th 2012, 01:57 PM
DeltaHax
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 :/
• Dec 11th 2012, 02:09 PM
skeeter
Re: Quadratic Equations
$(5x+10)^2 = 40$

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

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

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

$x = -2 \pm \frac{2\sqrt{10}}{5}$
• Dec 11th 2012, 02:35 PM
Prove It
Re: Quadratic Equations
Quote:

Originally Posted by DeltaHax
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 \begin{align*} \frac{-100 \pm \sqrt{10 \, 000 - 6000}}{50} \end{align*}, which then simplifies to

\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.