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Here is how I have worked it out so far:

$\log _2(x-4)+\log(x+2)=4$

$\log _2((x-4)(x+2)) = 4$

$(x-4)(x+2)=2^4$

$(x-4)(x+2)=16$

How do I proceed from here?

$x^2+2x-8 = 16$

$x^2+2x = 24$

$x(x+2) = 24$ Which I know is not the right answer

$x^2+2x-24 = 0$ Can't factor this

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  • $\begingroup$ Yes it makes it alot easier to help when you show the work like this. $\endgroup$ Nov 18, 2010 at 0:55
  • $\begingroup$ Something like this in mathematica simply checks for error :Solve[Log[2, x - 4] + Log[2, x + 2] == 4, x] $\endgroup$
    – Quixotic
    Nov 18, 2010 at 9:58

3 Answers 3

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It is $x^2-2x-8 = 16$ my friend. So you get $x^2 - 2x -24 = 0$, which factors as $(x-6)(x+4) = 0$. Hence, $x=6$ or $x = -4$.

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    $\begingroup$ I suspect that the x=-4 root is meant to be discarded (otherwise your claiming log_2(-8)+log_2(-2)=4). $\endgroup$ Nov 18, 2010 at 0:49
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    $\begingroup$ @ Douglas S. Stones: which is not totally false since I could take a branch of $\log_2(-8)$ and another branch of $\log_2(-2)$ to get $4$. But yes if you are talking about "conventional" logarithms, I need to discard $x=-4$. $\endgroup$
    – user17762
    Nov 18, 2010 at 7:29
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After $(x-4)(x+2)=16$, you get $x^2-2x-24=0$ (the coefficient of $x$ is $-2$ not $2$). So $x=\frac{2\pm \sqrt{100}}{2}$ by the quadratic formula. So $x=6$ or $x=-4$

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  • $\begingroup$ In the square root, how do you get 100? 4 - (4)(1)(24) = 4 - (96) = 92 $\endgroup$
    – rcapote
    Nov 18, 2010 at 0:24
  • $\begingroup$ $4 - (4)(1)(-24)=100$. The constant is $-24$, not $24$ $\endgroup$ Nov 18, 2010 at 0:27
  • $\begingroup$ Ah, I get it. Negative numbers are going to be the end of my math career :P $\endgroup$
    – rcapote
    Nov 18, 2010 at 0:32
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The quadratic formula: $$ x = \frac{ -b \pm \sqrt{b^2 - 4 a c} }{2a} = \frac{-2 \pm \sqrt{2^2 - 4 \times 1 \times (-24) }}{2 \times 1} $$ 4 + 96 in the square root.

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