how to find out the correct coefficients from a roots -vector
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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
댓글 수: 1
Jan
2012년 11월 16일
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.
답변 (2개)
Matt Fig
2012년 11월 16일
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
댓글 수: 0
Ahsan Zamee
2015년 2월 18일
poly is not used to find the roots rather it is used to build polynomials.
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