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..

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:

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

Now, trying -2, we find:

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

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

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

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

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

$\displaystyle x=\frac{2\pm2\sqrt{7}}{2}=1\pm\sqrt{7}$

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

Quote:

Originally Posted by

**MarkFL2**

Now, trying -2, we find:

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

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

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

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

What theorem is that? I think I missed the lecture over that theorem...

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.)

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.

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

Oh.... alright...

That clears it up! Many thanks!