How to solve a third order polynom?

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ChristianM89
ChristianM89 2018-8-22
评论： Star Strider 2018-8-22
I need some help solving a third order polynom. I have the function showen in the code. And I would like to get x. The first solution i expect is 30 for the code below.
Later I would like to solve e.g.: "fun == 200" and I expect the result to be 10.859
syms x;
fun = (pi*(x^3 - 234*x^2 + 18252*x))/2700;
eqn1 = fun == (674*pi)/5; %original Function
eqn2 = fun == 423.4867; %rounded
solve(eqn1,x)
30
102 - 3^(1/2)*24i
102 + 3^(1/2)*24i
solve(eqn2,x)
root(4398046511104*z^3*pi - 1029142883598336*z^2*pi + 80273144920670208*z*pi - 5028788349271654875, z, 1)
root(4398046511104*z^3*pi - 1029142883598336*z^2*pi + 80273144920670208*z*pi - 5028788349271654875, z, 2)
root(4398046511104*z^3*pi - 1029142883598336*z^2*pi + 80273144920670208*z*pi - 5028788349271654875, z, 3)
  1 个评论
Dimitris Kalogiros
Dimitris Kalogiros 2018-8-22
It seems that you can solve a polynomial equation of 3rd degree. What exactly your question is ?

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采纳的回答

Star Strider
Star Strider 2018-8-22
Use the related vpasolve function for ‘eqn2’:
vpasolve(eqn2,x)
30.000001280214526389167507957467
101.99999935989273680541624602127 - 41.56921827295475289777603563479i
101.99999935989273680541624602127 + 41.56921827295475289777603563479i

更多回答（1 个）

Torsten
Torsten 2018-8-22
Use MATLAB'S "roots".
Best wishes
Torsten.

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