# Can't figure out how.. (Pre-Calc, Rational Zero Theorem and Synthetic Division

• November 13th 2012, 11:51 AM
nojnotnow
Can't figure out how.. (Pre-Calc, Rational Zero Theorem and Synthetic Division
I've encountered the problem:

x^3 - 10x - 12

I am suppose to;
a.) list all possible rational roots
b.) use synthetic division to test
c.) find an actual root and use the quotient I found in part "b" to find the remaining roots and solve

So here's what happened:
1. I found all of the possible rational zeros
2. I used synthetic division on each possible root, none of which equaled zero.
3. For part "b" I assumed that there is no zero for this problem...
4. I check the answer in the back of my textbook and found the answer to "b" :
b.) -2
and c.) {-2, 1 + √7, 1 - √7}

I've done all I possibly can with what I know and I couldn't come up with the same answer in the textbook..
• November 13th 2012, 12:18 PM
MarkFL
Re: Can't figure out how.. (Pre-Calc, Rational Zero Theorem and Synthetic Division
Since the leading coefficient is 1, the list of possible rational roots will be integral:

$\pm(1,2,3,4,6,12)$

Now, trying -2, we find:

$f(-2)=(-2)^3-10(-2)-12=-8+20-12=0$

So, we know $x+2$ is a factor. Performing the division, we find:

$\frac{x^3-10x-12}{x+2}=x^2-2x-6$ and so:

$x^3-10x-12=(x+2)(x^2-2x-6)$

We know $x=-2$ is one root, the other two are found from the roots of the quadratic factor:

$x=\frac{2\pm2\sqrt{7}}{2}=1\pm\sqrt{7}$
• November 13th 2012, 12:30 PM
nojnotnow
Re: Can't figure out how.. (Pre-Calc, Rational Zero Theorem and Synthetic Division
Quote:

Originally Posted by MarkFL2

Now, trying -2, we find:

$f(-2)=(-2)^3-10(-2)-12=-8+20-12=0$

So, we know $x+2$ is a factor. Performing the division, we find:

$\frac{x^3-10x-12}{x+2}=x^2-2x-6$ and so:

$x^3-10x-12=(x+2)(x^2-2x-6)$

What theorem is that? I think I missed the lecture over that theorem...
• November 13th 2012, 12:38 PM
nojnotnow
Re: Can't figure out how.. (Pre-Calc, Rational Zero Theorem and Synthetic Division
I'm not sure if you saw my response, (I'm new at the moment, my apologies.)
• November 13th 2012, 01:07 PM
MarkFL
Re: Can't figure out how.. (Pre-Calc, Rational Zero Theorem and Synthetic Division
If r is a root of f(x), then f(r) = 0.
• November 13th 2012, 01:23 PM
nojnotnow
Re: Can't figure out how.. (Pre-Calc, Rational Zero Theorem and Synthetic Division
Oh.... alright...
That clears it up! Many thanks!