MHB Solution for Roots of x^3-7x+7=0: Week #340 Nov 13th, 2018 POTW

  • Thread starter Thread starter anemone
  • Start date Start date
AI Thread Summary
The discussion centers on evaluating the expression 1/a^4 + 1/b^4 + 1/c^4 for the roots a, b, and c of the polynomial equation x^3 - 7x + 7 = 0. Participants share their solutions, with castor28 providing a primary solution and Olinguito offering an alternative approach. Other members, including lfdahl and kaliprasad, are acknowledged for their correct solutions. The thread emphasizes collaboration and problem-solving within the math community. The evaluation of the expression highlights the relationships between the roots and their powers.
anemone
Gold Member
MHB
POTW Director
Messages
3,851
Reaction score
115
Here is this week's POTW:

-----

Given that $a,\,b$ and $c$ are roots for the equation $x^3-7x+7=0$.

Evaluate $\dfrac{1}{a^4}+\dfrac{1}{b^4}+\dfrac{1}{c^4}$.

-----

Remember to read the https://mathhelpboards.com/showthread.php?772-Problem-of-the-Week-%28POTW%29-Procedure-and-Guidelines to find out how to https://mathhelpboards.com/forms.php?do=form&fid=2!
 
Physics news on Phys.org
Congratulations to the following members for their correct solution:

1. castor28
2. Olinguito
3. lfdahl
4. kaliprasad

Solution from castor28:
Let us write $u=\dfrac1a$, $v = \dfrac1b$, $w = \dfrac1c$, $t(n) = u^n + v^n + w^n$. We want to find $t(4)$.

$u$, $v$, and $w$ are roots of the reciprocal equation $7x^3 - 7x^2 + 1 = 0$. $u^n$, $v^n$, $w^n$ and therefore $t(n)$ satisfy the corresponding linear recurrence relation $7x_n= 7x_{n-1} - x_{n-3}$.

We have $t(0) = 3$ (obvious), $t(1) = 1$ (by Viete's relations). We compute:
\begin{align*}
t(2) &= u^2 + v^2 + w^2\\
&= (u+v+w)^2 - 2(uv+uw+vw)\\
&= 1^2 - 2\cdot0\\
&= 1
\end{align*}

We can now use the recurrence to compute:
\begin{align*}
t(3) &= t(2) - \frac{t(0)}{7} = 1 - \frac37 = \frac47\\
t(4) &= t(3) - \frac{t(1)}{7} = \frac47 - \frac17 = {\bf \frac37}
\end{align*}

Alternative solution from Olinguito:
$a,b,c$ are roots of $x^3-7x+7=x(x^2-7)+7=0$

$\implies\ a^2,b^2,c^2$ are roots of $\sqrt x(x-7)+7=0$

$\implies$ they are roots of $x(x-7)^2=49$

$\implies$ they are roots of $x^3-14x^2+49x-49=0$

$\implies$ they are roots of $x(x^2+49)=14x^2+49$

$\implies\ a^4,b^4,c^4$ are roots of $\sqrt x(x+49)=14x+49$

$\implies$ they are roots of $x(x+49)^2=(14x+49)^2$

$\implies$ they are roots of $x^3-98x^2+1029x-2401=0$

$\implies\ \dfrac1{a^4},\dfrac1{b^4},\dfrac1{c^4}$ are roots of $\dfrac1{x^3}-\dfrac{98}{x^2}+\dfrac{1029}x-2401=0$

$\implies$ they are roots of $2401x^3-1029x^2+98x-1=0$

$\implies \dfrac1{a^4}+\dfrac1{b^4}+\dfrac1{c^4}=\dfrac{1029}{2401}=\boxed{\dfrac37}$.
 
Back
Top