# Homework Help: Roots of polynomials

1. Jan 23, 2008

### rock.freak667

1. The problem statement, all variables and given/known data

The roots of the equation $x^3-x-1=0$ are $\alpha,\beta,\gamma$
$S_n=\alpha^n +\beta^n +\gamma^n$

(i)Use the relation y=x$^2$ to show that $\alpha^2,\beta^2,\gamma^2$
are roots of the equation
$y^3-2y^2+y-1=0$
(ii)Hence, or otherwise find the value of $S_4$
(iii)Find $S_8,S_{12},S_{16}$
2. Relevant equations

$$\sum \alpha=\frac{-b}{a} \ \sum \alpha\beta=\frac{c}{a} \ \sum \alpha\beta\gamma=\frac{-d}{a}$$

3. The attempt at a solution

Just need help with the first part for now. Using I substituted y=x^2 into the equation they gave me in hopes to get back the original equation but that did not work out. Should I just find the sum of the roots and then the sum of the squares of the roots for the original equation and then find the sum of the roots for the eq'n in y and show that they are equal?

2. Jan 24, 2008

### symbolipoint

Are you permitted to use rational roots theorem, and fundamental theorem of algebra? The way you ask the question gives the impression that you are studying something beyond PreCalculus.

3. Jan 24, 2008

### HallsofIvy

You don't need anything deep like that. If y= x2, then it is easy to show that y3- 2y2+ y- 1= x6- 2x4+ x2- 1= (x3-x-1)(x3-x+ 1). Since $\alpha$, $\beta$, and $\gamma$ make the first factor 0, they make the entire polynomial 0.

Last edited by a moderator: Jan 24, 2008
4. Jan 24, 2008

### rock.freak667

oh thanks. No wonder I was confused. Shall attempt the other parts in a while.

5. Jan 24, 2008

### symbolipoint

I later did try what I suggested and found that the typical methods of "College Algebra" did not give ready/quick solutions. I found a real zero of about 1.3244.... something, and two complex zeros. I have never before seen that alpha, beta, gamma stuff. Again, what course is that from? Is it something beyond PreCalculus?

6. Jan 24, 2008

### HallsofIvy

You would use "rational roots" theorem and "Fundamental Theorem of Algebra" to try to FIND roots to the equation. Here you are already given the roots and want to show that they are roots. It's not as complicated as you seem to think!

7. Jan 24, 2008

### symbolipoint

I tried synthetic division for possible roots of +1 and -1, and they gave remainders. I then resorted to a graphing calculator and found a real root (not certain if it is rational or irrational) of about 1.3224. Using this value and again synthetic division, I came to two complex zeros or in any case, just resulting quadratic factor.

I could not pick any better rational values to test. According to the textbook of Precalculus, we pick using factors of the constant term and coefficient of the leading term, the positive and the negative values. These are based on integers.

8. Jan 24, 2008

### HallsofIvy

Apparently you simply did not understand what the question was asking! It is obvious, from the "rational root theorem" you cite, that neither of these equations has rational roots! The problem did not ask us to find any rational roots, or find any roots at all. It only asked us to show that roots of one equation must satisfy the other. That true because the given 6th degree polynomial (after substituting x2 for y) has the first 3rd degree polynomial equations as a factor.