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anonym
0

how to find out the correct coefficients from a roots -vector

Asked by anonym
on 16 Nov 2012
Latest activity Answered by Ahsan Zamee on 18 Feb 2015

Hi everbody Sry for my English ;)

I have a Problem mit those Matlab Functions : poly and roots for example, i have vector a with the polynomial coefficents

a =

     2     4     6     8

The roots of these Polynom is roots(a) =

              -1.6506          
              -0.1747 + 1.5469i
              -0.1747 - 1.5469i

The poly matlab function can be used to find out the coefficients from roots

but when i use ist on my roots(a), i didnt become my vector a again.

ans =

    1.0000    2.0000    3.0000    4.0000

Is there another Methode to find out the correct coefficents from roots ?

Many thanks

  1 Comment

Please post the code you use. The text description "poly can be used to find out the coefficients" does not allow to estimate, if you made a mistake when calling poly().

Currently I find in your question:

a = [2, 4, 6, 8];
roots(a)
>> -1.6506          
   -0.1747 + 1.5469i
   -0.1747 - 1.5469i
% Now poly() is mentioned
roots(a)
>> ans =
  1.0000    2.0000    3.0000    4.0000

I do not see, what's going on.

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2 Answers

Answer by Loginatorist on 16 Nov 2012

anonym, you need to think about what you are doing. When you have:

a = [2 4 6 8];

this represents this polynomial:

2*x^3 + 4*x^2 + 6*x + 8 = 0

Now what happens if you divide both sides by 2?? MATLAB simply returns this reduced form:

poly(roots(a)) 
ans =
      1.0000    2.0000    3.0000    4.0000

  0 Comments

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Answer by Ahsan Zamee on 18 Feb 2015

poly is not used to find the roots rather it is used to build polynomials.

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