# Havin the problem with solving a nonlinear overdetermined system of equations

I am having a problem to solve a nonlinear overdetermined system of equations. According to one of the previous posts, I tried NMinimize, but it didn’t work. Shall I use any other method? I will appreciate if somebody can give me some helps. This is what I did:

eqn1=c+a Cosh[(20048005 d)/80227844]==9;
eqn11=5000 c==21;
eqn2=1/400 (127-10 d)2 a+(127/20-d/2) b+c==1330711849393/15087435934161;
eqn3=10627269662 b==2478589 a Sinh[(20048005 d)/80227844];
eqn4=6400000 a==373 a Cosh[(20048005 d)/80227844];
eqn5=(127/10-d) a+b==2321294/1027365129;
eqn6=4009601 (95161+1270 d (-9+c))+20377872376 a Sinh[(20048005 d)/80227844]==0;


I considered one of the equations as a constrain, also I know that $0<=d<=12.7$ and all unknown are Reals.

NMinimize[{eqn1,eqn11,eqn2,eqn3,eqn4,eqn5&&eqn6&&0<= d<= 12.7&&{a,c,a,b,c,d}∈Reals},{a,c,a,b,c,d}]

NMinimize::nnum: The function value False is not a number at {d,a,a,b,c,c} = {11.7176,0.212654,0.277116,0.987641,-0.332112,0.0042}. >>
NMinimize::nnum: The function value False is not a number at {d,a,a,b,c,c} = {11.7176,0.212654,0.277116,0.987641,-0.332112,0.0042}. >>
NMinimize::nnum: The function value False is not a number at {d,a,a,b,c,c} = {11.7176,0.212654,0.277116,0.987641,-0.332112,0.0042}. >>


NMinimize requires a function rather than a list of equations.

Let's take your equations and try three things.

1. Use eqn11 to solve for c3. Replace c3 with 21/5000 in the other equations.

2. Replace the equality in the remaining equations with a subtraction.

eqn1 = c2 + a2 Cosh[(20048005 d)/80227844] - 9;

eqn2 = 1/400 (127 - 10 d) 2 a3 + (127/20 - d/2) b3 + 21/5000 - 1330711849393/15087435934161;

eqn3 = 10627269662 b3 - 2478589 a2 Sinh[(20048005 d)/80227844];

eqn4 = 6400000 a3 - 373 a2 Cosh[(20048005 d)/80227844];

eqn5 = (127/10 - d) a3 + b3 - 2321294/1027365129;

eqn6 = 4009601 (95161 + 1270 d (-9 + c2)) + 20377872376 a2 Sinh[(20048005 d)/80227844] - 0;

3. Reformulate the NMinimize into a function which is the sum of the square of the difference between the left hand and right hand side of your equations.

NMinimize[{
eqn1^2 + eqn2^2 + eqn3^2 + eqn4^2 + eqn5^2 + eqn6^2,
0 <= d <= 12.7 && {a2, c2, a3, b3, d} ∈ Reals
},
{a2, c2, a3, b3, d}]

(* {31.4845, {a2 -> 0.844706, c2 -> -0.921421, a3 -> 0.000251246, b3 -> 0.000985941, d -> 9.25743}} *)


In the formulation I took the liberty of replacing a with a2, c with c2, etc., which I found easier to follow.

• Thanks for your professional answer. But eqn1, eqn2 and eqn3 didn't satisfy?? Would you please give me some tips or comments about solving a nonlinear overdetermined system of equations. For example, What other methods do you recommend for solution such as FindFit[], NonlinearRegress[], etc. Feb 6 '16 at 7:12
• With an overdetermined set of equations one should not expect all equations to evaluate to zero. They are minimized in a least square sense. If you go to the help page for NMinimize and look at the top center, there is a pull down menu for Tutorials. There are four great tutorials there explaining the Mathematica optimizations that are available. Feb 6 '16 at 15:31