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I know this is a stupid question but, I posted this thread up not long a go Function derivatives. I'm trying to replicate this question. However, I can't seem to work out how I got the value $-2$ and $3$ from the quadratic formula. I've tried and tried to use the template, however each time im getting different values to the ones I got when I successfully done it. The formula is $6x^2-6x-36\dots$

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user120993
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No need to use the quadratic formula, but of course you can do so to yield the same roots:

$$\begin{align} 6x^2 - 6x - 36 = 0 & \iff 6(x^2 - x - 6) = 0 \\ \\ & \iff x^2 - x - 6 = 0\\ \\ & \iff (x+2)(x - 3)= 0 \\ \\ &\iff x = -2 \;\text{or}\; x = 3\end{align}$$

amWhy
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  • Thanks but I struggle doing all that you do. I know it's ironic since I can't find the answer :P but I much prefer the quadratic formula I just don't seem to be getting the right answer. If I post my working out will you show me where im going wrong? – user120993 Jan 13 '14 at 13:01
  • Using the quadratic formula, we have $a = 1, b = -1, c = -6$. $$\frac{-b \pm\sqrt{b^2 - 4ac}}{2a} = \frac{-(-1) \pm \sqrt{(-1)^2 - 4(1)(-6)}}{2(1)} = \frac {1 \pm \sqrt{1 + 24}}{2}$$ $x = \frac{1\pm 5}{2}$: $x = \frac 62 = 3$ or $x = \frac {-4}{2} = -2$. – amWhy Jan 13 '14 at 13:10
  • Do you understand that $6x^2 - 6x - 36 = 0 \iff x^2 - x - 6 = 0$? We simply divide each side of the equation by $6$ to simplify. – amWhy Jan 13 '14 at 13:15
  • Wait I thought a = 6 b = -6 and c = -36 im really worrying, my exam is in a few hours! my friends can just type it in on their calculators and get the correct answers but I can't do it on mine....ive got the casio fx-83ms – user120993 Jan 13 '14 at 13:36
  • If you use $a = 6, b=-6, c = -36$, you'll get the same exact same answers! Just do the calculations carefully! Remember to accept answers that are helpful ;-) – amWhy Jan 13 '14 at 13:47
  • I've accepted it..but on my calculator, ive typed it 1000 times and don't get the same x values.. – user120993 Jan 13 '14 at 13:58