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I am sure there is a better strategy that someone smarter than me would use, but I am not that person.

I am trying to factor$$x^3 - 3x^2 - 4x + 12 .$$

I do not know how, so I attempt to guess with long division. I cheat and look at the answer so find out one of the factors to save myself time, so I try $x - 2$.

I am not sure how to type out long division, but I get$$ (x - 2) \mid (x^3 - 3x^2 - 4x + 12) .$$

So I know I can have an $x^2$ for how many times $x$ goes into the leading term.

Subtracting it all, I am left with$$ (x - 2 ) \mid (-x^2 - 4x + 12) .$$

I know that the leading term goes into the inner leading term $-x$ times or however you say that.$$ (x - 2 ) \mid (- 2x + 12) .$$

Now $-2$:$$ (x - 2 ) \mid 8 .$$

Now I do not know what to do, how did this go so wrong? I have $x^2 - x -2$ on top, and I have a remainder of $8$. This cannot be right; I cheated, so I know that this should be a factor.

Later
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    The easiest way is: $( x^3 - 3x^2 ) - ( 4x - 12 ) = x^2 ( x - 3 ) - 4 ( x - 3 ) = ( x^2 - 4 ) ( x - 3 )$ – A.S May 18 '13 at 03:43
  • $x - 2 | - 2x + 12$ should be $x - 2 | - 6x + 12$. There are 3 minuses at that step :) – N. S. May 18 '13 at 03:48

3 Answers3

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Your first term, upon division: $x^2$ is correct $\large\checkmark$ (in the quotient), leaving

$$ x - 2 | -x^2 - 4x + 12\quad\large\checkmark$$

I know that the leading term goes into the inner leading term $-x$ times or however you say that.

$$ x - 2 | - 2x + 12 \quad \longleftarrow \text{error}$$

Here you subtracted incorrectly: We should have $x^2 - x$ in the quotient, that's correct, but multiplying $-x(x - 2) = -x^2 + 2x$

So when we subtract, we subtract, from $(-x^2 - 4x + 12) - (-x^2 + 2x) = -6x + 12$.

Now, we have $$ x -2 \mid -6x + 12$$ and so our ongoing quotient becomes $x^2 - x {\bf - 6}$

which, leaves a zero remainder since $-6(x-2) = -6x + 12$, as desired.

So...we have that $$\frac{x^3 - 3x^2 -4x + 12}{x - 2} = x^2 - x - 6$$

Or, that is, $$x^3 - 3x^2 - 4x + 12 = (x -2)(x^2 - x - 6) = (x-2)(x +2)(x - 3)$$

Community
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amWhy
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When you subtracted $(-x)(x-2)$ you should have gotten $-6x+12$. You dropped a sign there.

Ross Millikan
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As Andrew Salmon's comment points out, the easiest thing to try first to factor a cubic polynomial is to try to pair off the terms and see if they share a common factor. If paired off in the order in which they are written, this becomes

$$x^2(x-3)-4(x-3)$$

or if the odd and even exponents are paired together

$$x(x^2-4)-3(x^2-4)$$

Then simply take out the common factor.

Of course, this won't always work, when it doesn't, the method of guessing roots is known as the rational root theorem. First you'll want to try plugging in factors of the constant term to see if you get zero. $\pm1$ is the easiest to check, then in this case you'd also have $\pm2,\pm3,\pm4,\pm6$ and $\pm12$. If those all failed, you'd want to test any of those values divided by a factor of the leading term. However, since your leading term is $1$, there isn't anything extra to test here. If you find one root, you should be able to handle the remaining quadratic. I doubt you'll be expected to factor any cubic polynomials with no rational roots, possibly excepting the form

$$x^3\pm a$$

at which point you could use the formula for the sum or difference of cubes even if $a$ is not a perfect cube.

Finally, although it's not foolproof, if you suspect a polynomial is a multiple of $x-2$, then plugging in $2$ should yield $0$. Since you're dividing by $x-2$, your remainder at each step should also be a multiple of $x-2$. Testing your polynomials at each step reveals

$$2^3-3(2)^2-4(2)+12=8-12-8+12=0$$ $$-(2)^2-4(2)+12=-4-8+12=0$$ $$-2(2)+12=-4+12=8$$

This implies there was an error in the remainder $-2x+12$

Mike
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  • I don't really follow the first two things you did, it doesn't look correct and I can't make sense of it. If you factor out terms from two different sets you can just rearrange them in any order? –  May 18 '13 at 14:29
  • @Jordan You can rearrange sums in any order. And factoring is basically just using the distributive property in reverse. So for the first one I factored out $x^2$ from the first 2 terms and $-4$ from the last 2. The next step would be to factor out $x-3$ to get $(x-3)(x^2-4)$. – Mike May 18 '13 at 14:39