MHB Roots of Cubic Equation: Finding $x_1,\,x_2$, and $x_3$

  • Thread starter Thread starter anemone
  • Start date Start date
  • Tags Tags
    Cubic Roots
anemone
Gold Member
MHB
POTW Director
Messages
3,851
Reaction score
115
The roots $x_1,\,x_2$ and $x_3$ of the equation $x^3+ax+a=0$ where $a$ is a non-zero real number, satisfy $\dfrac{x_1^2}{x_2}+\dfrac{x_2^2}{x_3}+\dfrac{x_3^2}{x_1}=-8$. Find $x_1,\,x_2$ and $x_3$.
 
Mathematics news on Phys.org
We are given the following:
$x_1^3x_3+x_2^3x_1+x_3^3x_2=-8x_1x_2x_3,\\x_1+x_2+x_3=0,\\x_1x_2+x_2x_3+x_3x_1=a,\\x_1x_2x_3=-a$ and for $i=1,\,2,\,3$, $x_i^3+ax_i+a=0$.

Now

$x_1^3+ax_1+a=0\\x_2^3+ax_2+a=0\\x_3^3+ax_3+a=0$ gives

$(x_1^3x_3+x_2^3x_1+x_3^3x_2)+a(x_1x_3+x_2x_1+x_3x_2)+a(x_3+x_2+x_1)=0$

i.e. $8a+a^2=0,\implies a=-8$.

So the given equation is $x^3-8x-8=0$. One root is $-2$ and the other roots are given by $x^2-2x-4=0$, i.e. $x=1\pm \sqrt{5}$.
 
Insights auto threads is broken atm, so I'm manually creating these for new Insight articles. In Dirac’s Principles of Quantum Mechanics published in 1930 he introduced a “convenient notation” he referred to as a “delta function” which he treated as a continuum analog to the discrete Kronecker delta. The Kronecker delta is simply the indexed components of the identity operator in matrix algebra Source: https://www.physicsforums.com/insights/what-exactly-is-diracs-delta-function/ by...
Fermat's Last Theorem has long been one of the most famous mathematical problems, and is now one of the most famous theorems. It simply states that the equation $$ a^n+b^n=c^n $$ has no solutions with positive integers if ##n>2.## It was named after Pierre de Fermat (1607-1665). The problem itself stems from the book Arithmetica by Diophantus of Alexandria. It gained popularity because Fermat noted in his copy "Cubum autem in duos cubos, aut quadratoquadratum in duos quadratoquadratos, et...
I'm interested to know whether the equation $$1 = 2 - \frac{1}{2 - \frac{1}{2 - \cdots}}$$ is true or not. It can be shown easily that if the continued fraction converges, it cannot converge to anything else than 1. It seems that if the continued fraction converges, the convergence is very slow. The apparent slowness of the convergence makes it difficult to estimate the presence of true convergence numerically. At the moment I don't know whether this converges or not.

Similar threads

Replies
2
Views
1K
Replies
5
Views
2K
Replies
2
Views
2K
Replies
1
Views
946
Replies
1
Views
1K
Replies
3
Views
1K
Back
Top