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I need to solve an polynimial equation, but when i try to use Solve or NSolve in mathematica, its cant find the solution in appropriate time (i interrupted calculation after 5 hour left). The equations is:

Remove["Global`*"];
g = 9.8;
C1 = 3.6;
C2 = 0.25;
C3 = 666;
C4 = 2;
c = -36;
e = 0.86;
P0 = 24.44;
h0 = 0.1;

h = h0*(1 + C3*P^C4);
NSolve[{P/P0 == ((1 - e*(1 - h/e))/(1 - e))^((2 + c)/3)/(1 - h/e)^4.65}, {P}]

Maybe my computer is too weak or I'm using the wrong method?

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I should check the code before publication, I just tried to move only part that was a problem, I corrected value for x0 I do not quite understand "Also using the dependent variable dP in the RHS of h in the wrong way." - I can express from the first equation the variable P or h and make a single equation, but the result is the same. I apologize if I do not understand some obvious things I have enough experience in dealing with mathematics –  user1058051 Jan 8 at 4:37
    
@Nasser: NDsolve? Unless the post changed, that's not being used. OP: Nasser's points are spot-on - if this is some kind of HW problem, please specify such, and perhaps add some context of the over-arching problem. The MM solvers are not magic machines, you might need to 'help' them. Hint: try plotting your function with P around 0.02368. –  rasher Jan 8 at 4:51
    
@Nasser Wow, thanks very much, It works. –  user1058051 Jan 8 at 14:17
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1 Answer

up vote 3 down vote accepted

Ok, try this to see if works for you. Made a function in P (wish you used lowerLetterCase) and plotted it. For some values of P, the function is complex valued.

One can see the roots at around 0 and .1. Hence use FindRoot to pick them up.

Clear[g, C1, C2, C3, C4, c, e, P0, h0, exp, P]
parms = {g -> 9.8, C1 -> 3.6, C2 -> 0.25, C3 -> 666, C4 -> 2, c -> -36, e -> 0.86, 
        P0 -> 24.44, h0 -> 0.1, exp -> 4.65};
h = h0*(1 + C3*P^C4);
f[P_?NumericQ] := P/P0 - ((1 - e*(1 - h/e))/(1 - e))^((2 + c)/3)/(1 - h/e)^exp;
(*verify*)
Simplify[f[P] /. parms]

Mathematica graphics

Not a nice looking function in P. If you can simply it a little it will help. You can see the numerator on the second term is almost zero. The limit of the second term as P->0 is 0.0039, so may be you can simplify this whole function to a linear function (straight line) in P around zero.

Here is a plot of f[P]

Plot[Re[f[P] /. parms], {P, -.25, .25}, Exclusions -> None]

Mathematica graphics

Plot[Im[f[P] /. parms], {P, -.25, .25}, Exclusions -> None]

Mathematica graphics

use FindRoot

FindRoot[(f[P] /. parms) == 0, {P, .01}]
(* {P -> 0.0236849} *)

FindRoot[(f[P] /. parms) == 0, {P, .11}]
(* {P -> -0.0256356 + 0.0197882 I} *)

FindRoot[(f[P] /. parms) == 0, {P, -.11}]
(* {P -> 0.0236849 + 1.34501*10^-27 I}  *)

may be someone will see a better way to handle this.

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