Solve a system of 3 equations with 3 unknowns

Pavel (view profile)

on 4 Apr 2014
Latest activity Commented on by Pavel

on 5 Apr 2014

Walter Roberson (view profile)

I have the folowing code. My 3 equations are diff_f1, diff_f2 and diff_f3 with the unkowns a,b,c. I need to make the equations equal to 0 and to solve for a,b and c, but when i do that i obtain that a=b=c=0 and that are not the answers i'm looking for. What i'm doing wrong and how i can solve these equations?

```if true
clear all
clc
% a=alpha1; b=alpha2; c=betha
syms a b c d N1 N2 N3 N21 N31 n2 n21 n3 l1 l2 l3 P L sa
% Se calculeaza lungimea elementelor
P=2000;
L=1300;
sa=150;
l1=L/cos(a);
l2=L/cos(b);
l3=L/cos(b+c);
d=a+b;
% Se calculeaza A
N21=(-P*cos(b+c)/(cos(b)*sin(b+c)-cos(b+c)));
N31=P/(sin(b+c)-cos(b+c)/cos(b));
n31=(-sin(a)*cos(b)-sin(b)*cos(a))/(sin(b+c)*cos(b)-cos(b+c)*sin(b));
n3=n31;
n21=(cos(a)-n3*cos(b+c))/cos(b);
n2=n21;
%efortul din elementul 1
N1=(-((n2*l2*N21+n3*l3*N31)/(l1+n2^2*l2*n3^2*l3)));
%efortul din elementul 3
N3=(P*cos(b)-N1*sin(a)*cos(b)-N1*cos(a))/(cos(b)*(sin(b+c)-cos(b+c)));
%efortul din elementul 2
N2=(N1*cos(a)-cos(b+c)*N3)/(cos(b));
%definrea functiei obiectiv
V1=((N1/sa)*l1+(N2/sa)*l2+(N3/sa)*l3);
%diff(V(a,b,c),a)
V=simplify(V1);
diff_f1=diff(V,a);
diff_f2=diff(V,b);
```

diff_f3=diff(V,c);

[a b c]=solve('diff_f1=0','diff_f2=0','diff_f3=0');

```end
```

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Walter Roberson (view profile)

on 4 Apr 2014

```[a, b, c] = solve(diff_f1, diff_f2, diff_f3);
```

Pavel

Pavel (view profile)

on 5 Apr 2014

Thank you. I used this code:

```if true
S=vpasolve([diff_f1==0, diff_f2==0, diff_f3==0], [a b c],[-2*pi 2*pi])
end
but i get the following error:
"Undefined function 'vpasolve' for input arguments of type 'sym'.
"
```
Walter Roberson

Walter Roberson (view profile)

on 5 Apr 2014

Which MATLAB release are you using?

In the meantime try

```feval(symengine, 'numeric::solve', [diff_f1, diff_f2, diff_f3], [a b c], [-2*pi 2*pi])
```
Pavel

Pavel (view profile)

on 5 Apr 2014

i'm using R2012a

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