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Asked by anonym
on 16 Nov 2012

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

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Answer by Matt Fig
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

Answer by Ahsan Zamee
on 18 Feb 2015

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

## 1 Comment

## Jan Simon (view profile)

Direct link to this comment:http://www.mathworks.com/matlabcentral/answers/53887#comment_111486

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:

I do not see, what's going on.