What is the Polynomial Function with Given Zeros and Real Coefficients?

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Discussion Overview

The discussion revolves around finding a polynomial function of least degree with real coefficients given specific complex zeros, specifically \(5+i\) and \(4-i\). Participants explore the correct formulation of the polynomial and the necessary steps to derive it, focusing on the implications of complex zeros and their conjugates.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant proposes that the polynomial can be formed by multiplying the factors corresponding to the given zeros, initially calculating \((x-(5+i))(x-(5-i))\) and \((x-(4-i))(x-(4+i))\).
  • Another participant corrects the quadratic products, suggesting that the initial calculations are incorrect and that the correct form should include \(x\) terms.
  • A later reply confirms the need for the quadratics to be correct before forming the quartic polynomial as their product.
  • One participant states that a polynomial with real coefficients must include the conjugate pairs of the complex zeros, leading to a polynomial of degree at least 4.
  • Another participant provides a general form for a polynomial with complex zeros, indicating that the polynomial can be expressed as \(k(x^2-10x+26)(x^2-8x+17)\), where \(k\) is a nonzero real number.
  • One participant inquires about online calculators for finding zeros, indicating a search for additional tools or resources.

Areas of Agreement / Disagreement

Participants generally agree on the necessity of including conjugate pairs for complex zeros and the requirement for the polynomial to be of degree at least 4. However, there is disagreement regarding the correctness of the initial quadratic products, which remains unresolved.

Contextual Notes

There are unresolved issues regarding the accuracy of the quadratic products and the steps leading to the quartic polynomial. The discussion reflects varying levels of certainty about the calculations involved.

Who May Find This Useful

This discussion may be useful for students or individuals interested in polynomial functions, particularly those learning about complex zeros and their implications in polynomial formation.

karush
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Find a polynominal function f(x) of least degrees having only the real coeficents and zeros as given

$$5+i \quad 4-i$$
ok I did this but don't think this is the final answer
$(x-(5+i))(x-(5-i))=x^2+26$
$(x-(4-i))(x-(4+i))=x^2-17$
 
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You've got the right idea, but your quadratic products aren't correct. :)
 
MarkFL said:
You've got the right idea, but your quadratic products aren't correct. :)

$$(x-(5+i))(x-(5-i))=x^2+26$$
$$(x-(4-i))(x-(4+i))=x^2+17$$

is this as far as can go?

or is it

(x^2+26)(x^2+17)
 
You want to first get two quadratics as you've done, and then the quartic polynomial will be their product, but you don't have the correct quadratics yet. Check your multiplications...you should have \(x\) terms. :)
 
MarkFL said:
You want to first get two quadratics as you've done, and then the quartic polynomial will be their product, but you don't have the correct quadratics yet. Check your multiplications...you should have \(x\) terms. :)
$$(x^2 - 10 x + 26)(x^2 - 8 x +17)=x^4-18x^3+123x^2-378x+442$$
 
karush said:
Find a polynominal function f(x) of least degrees having only the real coeficents and zeros as given

$$5+i \quad 4-i$$
ok I did this but don't think this is the final answer
$(x-(5+i))(x-(5-i))=x^2+26$
$(x-(4-i))(x-(4+i))=x^2-17$

A polynomial with real coefficients will have complex zeroes occurring in conjugate pairs. So if two of the zeroes are $5+i$ and $4-i$, the other ones will be $5-i$ and $4+i$. Hence $f(x)$ will be a polynomial of degree at least 4.

A polynomial with zeroes $a\pm bi$ is $x^2-2ax+(a^2+b^2)$. Hence
$$f(x)\ =\ k(x^2-10x+26)(x^2-8x+17)$$
where $k$ is any nonzero real number.
 
What are some good online calculators for hunting down zeros

I thot EMath was nice and you
Pull latex from it.
 

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