MHB Find Solutions to a+b+c+d=4, a^2+b^2+c^2+d^2=6, a^3+b^3+c^3+d^3=94/9 in [0,2]

  • Thread starter Thread starter anemone
  • Start date Start date
AI Thread Summary
The discussion focuses on finding solutions to the system of equations involving the sums of variables a, b, c, and d, constrained within the interval [0, 2]. The equations are a+b+c+d=4, a^2+b^2+c^2+d^2=6, and a^3+b^3+c^3+d^3=94/9. Participants are encouraged to explore potential values for a, b, c, and d that satisfy all three equations simultaneously. The discussion emphasizes the importance of considering the specified range for each variable. The goal is to identify all valid combinations that meet these criteria.
anemone
Gold Member
MHB
POTW Director
Messages
3,851
Reaction score
115
Find all the solutions to the system

$a+b+c+d=4\\a^2+b^2+c^2+d^2=6\\a^3+b^3+c^3+d^3=\dfrac{94}{9}$
in $[0, 2]$.
 
Mathematics news on Phys.org
anemone said:
Find all the solutions to the system $p_1 = a+b+c+d=4\\p_2 = a^2+b^2+c^2+d^2=6\\ p_3 = a^3+b^3+c^3+d^3=\dfrac{94}{9}$ in $[0, 2]$.
Let $x^4 - e_1x^3 + e_2x^2 - e_3x + e_4 = 0$ be the equation with roots $a,b,c,d$. By Newton's identities, $$\textstyle e_1 = p_1 = 4,\qquad e_2 = \frac12(p_1^2 - p_2) = \frac12(16 - 6) = 5,\qquad e_3 = \frac16(p_1^3 - 3p_1p_2 + 2p_3) = \frac16(64 - 72 + \frac{188}9) = \frac{58}{27}.$$ So the equation is $x^4 - 4x^3 + 5x^2 - \frac{58}{27}x + e_4 = 0$. Since $58$ is close to twice $27$, write the equation as $$x^4 - 4x^3 + 5x^2 - 2x = \tfrac4{27}x - e_4, \\ x(x-2)(x^2 - 2x + 1) = \tfrac4{27}(x-s),$$ where $s$ is a constant. Now look at the graph:

[DESMOS]{"version":7,"graph":{"viewport":{"xmin":-0.47946479454949004,"ymin":-1.7930541038513184,"xmax":2.51919243201301,"ymax":1.9552674293518066}},"randomSeed":"5a5979bd9ad99a52fd1cfa25e8de4160","expressions":{"list":[{"type":"expression","id":"1","color":"#c74440","latex":"x^{4}-4x^{3}+5x^{2}-2x"},{"type":"expression","id":"2","color":"#2d70b3","latex":"\\frac{4}{27}\\left(x-s\\right)"},{"type":"expression","id":"4","color":"#6042a6","latex":"s=2","hidden":true,"slider":{"hardMin":true,"hardMax":true,"min":"1","max":"3","step":"0.1"}},{"type":"expression","id":"5","color":"#000000"}]}}[/DESMOS]
The roots of the equation are the points where the blue line meets the red curve. By using the slider, you can see that if $s<2$ then the largest root is greater than $2$. But if $s>2$ then the blue line goes lower, and only meets the red curve in two points, which means that two of the roots of the quartic equation are complex. So for the equation to have four real roots in the interval $[0,2]$, $s$ must be equal to $2$. After multiplying by $27$ the equation then becomes $(x-2)(27x^3 - 54x^2 + 27x - 4) = 0$, which factorises as $(3x-1)^2(3x-4)(x-2) = 0$. Therefore the solutions to the system are $\{a,b,c,d\} = \{\frac13,\frac13,\frac43,2\}$ (in any order).
 
Awesome, Opalg!(Cool) And thanks for participating!

I will start from the quartic equation $p(x)=x^4-4x^3+5x^2-\dfrac{58}{27}x+k$ where $p(x)$ has roots $a, b, c, d$.

$p'(x)=4x^3-12x^2+10x-\dfrac{58}{27}=\dfrac{2}{27}(3x-1)(18x^2-48x+29)$

Solving $p/(x)=0$ gives $x=\dfrac{1}{3},\,\dfrac{4}{3}\pm\dfrac{\sqrt{6}}{2}$.

Since $p(x)$ is a 4th degree polynomial with positive leading coefficient and $p'(x)$ has 3 distinct real roots in $(0, 2)$, it follows that in order for $a, b, c, d$ to be solutions of the given equations where $0\le a, b, c, d\le 2$, we must have

$p(0)\ge 0,\,p\left(\dfrac{1}{3}\right)\le0,\,p\left(\dfrac{4}{3}-\dfrac{\sqrt{6}}{2}\right)\ge0,\, p\left(\dfrac{4}{3}+\dfrac{\sqrt{6}}{2}\right)\le0,\,p(2)\ge 0$

Evaluating, we find $p\left(\dfrac{1}{3}\right)=p(2)=k-\dfrac{8}{27}$. Hence, $k=\dfrac{8}{27}$, from which we obtain

$\begin{align*}p(x)&=x^4-4x^3+5x^2-\dfrac{58}{27}x+\dfrac{8}{27}\\&=\dfrac{1}{27}(27x^4-108x^3+135x^2-58x+8)\\&=\dfrac{1}{27}(3x-1)^2(3x-4)(x-2)\end{align*}$

Therefore, the solutions in $[0, 2]$ are the 12 permutations of $\left(\dfrac{1}{3},\, \dfrac{1}{3},\, \dfrac{4}{3},\,2 \right)$.
 
Suppose ,instead of the usual x,y coordinate system with an I basis vector along the x -axis and a corresponding j basis vector along the y-axis we instead have a different pair of basis vectors ,call them e and f along their respective axes. I have seen that this is an important subject in maths My question is what physical applications does such a model apply to? I am asking here because I have devoted quite a lot of time in the past to understanding convectors and the dual...
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...
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...

Similar threads

Replies
1
Views
1K
Replies
3
Views
1K
Replies
1
Views
994
Replies
2
Views
1K
Replies
7
Views
2K
Replies
9
Views
3K
Back
Top