Why is the polyval command giving two different answers?

1 visualización (últimos 30 días)
Baldemy
Baldemy el 22 de Sept. de 2014
Comentada: Matt J el 23 de Sept. de 2014
Why does the polyval operator not work as expected. Is the ans variable not stored as a column vector? Why aren't the second, fifth, and sixth results equal?
>> roots([1,-8,17,2,-24])
ans =
4.0000
3.0000
2.0000
-1.0000
>> polyval([1.-8,17,2,-24],ans)
ans =
-192.0000
-54.0000
-8.0000
-2.0000
>> roots([1,-8,17,2,-24])
ans =
4.0000
3.0000
2.0000
-1.0000
>> x=ans
x =
4.0000
3.0000
2.0000
-1.0000
>> polyval([1,-8,17,2,-24],x)
ans =
1.0e-13 *
0.8882
0.3197
0.0355
0.1421
>> polyval([1,-8,17,2,-24],[2.0000;3.0000;-1.0000;3])
ans =
0
0
0
0

Respuesta aceptada

Alberto
Alberto el 22 de Sept. de 2014
Instruction roots uses an iterative numeric method to approximate the solution in float arithmetic. What you get is an excellent approximation.
If you need the exact solution you should try a symbolic method:
g = x^4-8*x^3 + 17*x^2 +2*x -24
g =
x^4 - 8*x^3 + 17*x^2 + 2*x - 24
>> sol=solve(g==0)
sol =
2
3
4
-1
  1 comentario
Matt J
Matt J el 23 de Sept. de 2014
You also may need a symbolic version of polyval, even when you have the exact roots:
>> polyval([1,-8,17,2,-24]/3,[4 3 2 -1])
ans =
1.0e-14 *
0.8882 0.1776 0.1776 0.1776

Iniciar sesión para comentar.

Más respuestas (1)

Matt J
Matt J el 22 de Sept. de 2014
Editada: Matt J el 22 de Sept. de 2014
Because you have a typo in your call to polyval: a period appears where a comma should be.
  2 comentarios
Baldemy
Baldemy el 22 de Sept. de 2014
Editada: Matt J el 22 de Sept. de 2014
ok then this happens instead
>> roots([1,-8,17,2,-24])
ans =
4.000000000000009
2.999999999999992
2.000000000000001
-1.000000000000000
>> polyval([1,-8,17,2,-24],ans)
ans =
1.0e-13 *
0.888178419700125
0.319744231092045
0.035527136788005
0.142108547152020
Matt J
Matt J el 22 de Sept. de 2014
Editada: Matt J el 22 de Sept. de 2014
And what don't you like about it? The result is quite close to zero, as one would expect.

Iniciar sesión para comentar.

Categorías

Más información sobre Logical en Help Center y File Exchange.

Productos

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by