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Re: Problems with Solve
Posted:
Mar 26, 2014 3:23 AM


On Sunday, March 23, 2014 5:01:17 AM UTC4, totarefugium wrote: > Hello, > > In[1]:= N[Solve[z + 5 (z^2  1) + 1 z^3 == 1, z]] > > Out[1]= {{z > 0.925423 + 0. I}, {z > 4.47735 + 2.22045*10^16 I}, {z > > > 1.44807  4.44089*10^16 I
> > This equation has 3 real solutions. However, the answers returned when I ask mathematica for a decimal answer are complex (which I need to do later on when an integration needs solving numerically): > > > {{z > 0.925423 + 0. I}, {z > 4.47735 + > > > 2.22045*10^16 I}, {z > 1.44807  4.44089*10^16 I}}
(* A complex solution *)
sol=z/.Solve[z+5 (z^21)+1 z^3==1,z];
sol//ComplexExpand//FullSimplify
{1/3 (5+Sqrt[22] Cos[1/3 ArcTan[(9 Sqrt[503])/43]]+Sqrt[66] Sin[1/3 ArcTan[(9 Sqrt[503])/43]]), 1/3 (52 Sqrt[22] Cos[1/3 ArcTan[(9 Sqrt[503])/43]]), 1/3 (5+Sqrt[22] Cos[1/3 ArcTan[(9 Sqrt[503])/43]]Sqrt[66] Sin[1/3 ArcTan[(9 Sqrt[503])/43]])}
N[%,25]
0.9254229177777078310769981, 4.477352348008969997944065, 1.448070569768737833132933



