Hi Fabian,
From your code, I can understand that you have 7 variables, "x1", "x2", "x3", "x4", "x6", "x7" and "x8". You are first solving for "x1", "x2", "x3" and "x4" in terms of the other variables, and putting them back to solve for the rest 3 variables.
I tried to solve for "Abl_x6==0", "Abl_x7==0" and "Abl_x8==0" using your code, but with this approach, the runtime of the code is very high. This is because solving the first 4 variables in terms of rest 3 makes all these expressions very complex. Instead, you should put all the conditions together in one "solve()" statement. This lets MATLAB do internal optimizations for solving.
After putting all the equations together, it becomes a problem having 7 equations and 7 variables.
Note that "vpa()" only works if all the parameters are resolved in the equations. Otherwise, substitute the parameters with numbers by using "subs()" before calling "vpa()", or try using the "eval()" or the "simplify()" function.
Please refer to the following sample code:
syms x1 x2 x3 x4 x6 x7 x8
a1 = (x3-x1)/(2*x7)+x7/2;
a2 = (x2-x3)/(2*(x6-x7))+(x7+x6)/2;
a3 = (x4-x2)/(2*(x8-x6))+(x8+x6)/2;
Prof_A = x1*a1+x2*(a3-a2);
Prof_B = x3*(a2-a1)+x4*(1-a3);
Abl_x1 = diff(Prof_A,x1);
Abl_x2 = diff(Prof_A,x2);
Abl_x3 = diff(Prof_B,x3);
Abl_x4 = diff(Prof_B,x4);
Abl_x6 = diff(Prof_A,x6);
Abl_x7 = diff(Prof_B,x7);
Abl_x8 = diff(Prof_B,x8);
S = solve(Abl_x1 == 0, Abl_x2 == 0, Abl_x3 == 0, Abl_x4 == 0,Abl_x6 == 0, Abl_x7 == 0, Abl_x8 == 0);
ans_x6 = vpa(S.x6);
ans_x7 = vpa(S.x7);
ans_x8 = vpa(S.x8);
I hope this helps you address the issue.
