MHB Find Cubic Function & Polynomial of Degree 3

  • Thread starter Thread starter anemone
  • Start date Start date
  • Tags Tags
    Cubic Function
AI Thread Summary
A polynomial of degree 3 with real coefficients can be constructed where each root is the square of the roots of the polynomial P(x) = x^3 + 9x^2 + 9x + 9. The roots of P(x) are denoted as a, b, and c, leading to the new roots a^2, b^2, and c^2. By applying polynomial properties, the coefficients of the new polynomial P(y) can be determined as A = -63, B = -81, and C = -81. The resulting polynomial is P(y) = y^3 - 63y^2 - 81y - 81. This demonstrates a method for deriving a polynomial based on the squared roots of another polynomial.
anemone
Gold Member
MHB
POTW Director
Messages
3,851
Reaction score
115
Find a polynomial of degree 3 with real coefficients such that each of its roots is equal to the square of one root of the polynomial $P(x)=x^3+9x^2+9x+9$.
 
Mathematics news on Phys.org
Because the square of the roots shall be roots of the required polynomial so we must have $\sqrt x$ as roots of P(X)
So $(\sqrt x)^3 + 9 x + 9 (\sqrt x) + 9 = 0$
Or $(\sqrt x)(x + 9) = - 9(x+1)$
Or squaring $x(x+9)^2 = 81(x+1)^2$
Or $x(x^2 + 18 x + 81) = 81 x^2 + 162x + 81$
Or $x^3 - 63x^2 - 81 x - 81 = 0$
This is the required equation
 
anemone said:
Find a polynomial of degree 3 with real coefficients such that each of its roots is equal to the square of one root of the polynomial $P(x)=x^3+9x^2+9x+9$.
Does the above mean that if a,b,c are the 3 roots of the above polynomial then the the roots of the new polynomial must be $a^2 ,b^2 , c^2$ ??
 
kaliprasad said:
Because the square of the roots shall be roots of the required polynomial so we must have $\sqrt x$ as roots of P(X)
So $(\sqrt x)^3 + 9 x + 9 (\sqrt x) + 9 = 0$
Or $(\sqrt x)(x + 9) = - 9(x+1)$
Or squaring $x(x+9)^2 = 81(x+1)^2$
Or $x(x^2 + 18 x + 81) = 81 x^2 + 162x + 81$
Or $x^3 - 63x^2 - 81 x - 81 = 0$
This is the required equation

[sp]kaliprasand must we not have also $ -\sqrt x$ .Anyway i did this problem in a different way and i got the same answer [/sp]
 
solakis said:
[sp]kaliprasand must we not have also $ -\sqrt x$ .Anyway i did this problem in a different way and i got the same answer [/sp]

you are right and it is my mistake

as $\sqrt{x}$ is positive by definition so my line should be $\pm \sqrt{x}$ and the we get the same result because of squaring
 
[sp]
Let a,b,c be the roots of P(x) = $x^3+9x^2+9x+9$
And $a^2, b^2 ,c^2 $ the roots of P(y)= $y^3+Ay^2+By+C$
From the polynomial properties we have:
P(x) = $x^3+9x^2+9x+9$=(x-a)(x-b)(x-c) $\Rightarrow$...
.a+b+c=-9...... (1)
ab+ac+bc=9 ...(2)
abc=-9 ......(3)
And also P(y)= $y^3+Ay^2+By+C$=$(y-a^2)(y-b^2)(y-c^2)\Rightarrow $
$a^2+b^2+c^2=-A$..........(4)
$a^2b^2 +a^2c^2+b^2c^2=B$......(5)
$ a^2b^2c^2=-C$............(6)
From(3) we have:$a^2b^2c^2=81$ hence
C=-81.................(7)
From (1) and squaring we get :
$(a+b+c)^2=81\Rightarrow a^2+b^2+c^2= 81-2(ab+ac+bc) $ and using (2) we get :
$a^2+b^2+c^2= 63$ and hence :
A= -63...............(8)
From (2) and squaring we get:
$(ab+ac+bc)^2=81\Rightarrow a^2b^2+a^2c^2+b^2c^2+2(a^2bc+ab^2c+abc^2)=81\Rightarrow
a^2b^2+a^2c^2+b^2c^2+2(a(abc)+b(abc)+c(abc))=81$ and using (3) we have:
$a^2b^2+a^2c^2+b^2c^2 =81-2(-9a-9b-9c)=81+18(a+b+c)$ and using (1) we have:
$a^2b^2+a^2c^2+b^2c^2=81+18(-9)=-81$ hence:
B=-81...............(9)
And P(y)= $y^3+Ay^2+By+C =y^3-63y^2-81y-81$ by using (7),(8),(9)
[/sp]
 
Last edited by a moderator:
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...
Back
Top