# Solve non-linear symbolic equation

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mohamed faida am 30 Mai 2022
Kommentiert: Karan Singh am 9 Nov. 2023
i'm trying to find the solution of a non-linear equation using the command solve. the problem is a trigonometric one, where the variables are all dependent of the symbol T.
syms ('T','real');
cth = (1-T^2)/(1+T^2);
sth = 2*T/(1+T^2);
A=[G1 G2; G4 G5];
B=[-G3; -G6]; %Gi for i=1:6 is function of T
X = A\B;
x=X(1); %function of T
y=X(2); %function of T
Eq= (x-b1x)^2+(y-b1y)^2 - d^2;
T_values = solve(Eq,T);
T_values = vpa(T_values);
theta = 2*atan(T)
it shows Warning message: Solutions are only valid under certain conditions. To
include parameters and conditions in the solution, specify the
'ReturnConditions' value as 'true'.
then changing to:
T_values = solve(Eq,T,'ReturnConditions',true)
displays the following error: Error using sym>tomupad (line 1539)
Unable to convert 'struct' to 'sym'.
##### 3 Kommentare1 älteren Kommentar anzeigen1 älteren Kommentar ausblenden
Maneet Kaur Bagga am 13 Sep. 2023
Hi Mohamed,
It would be easy to reproduce the code and solve the error if you provide the values of the variables.
Walter Roberson am 9 Nov. 2023
syms ('T','real');
cth = (1-T^2)/(1+T^2);
sth = 2*T/(1+T^2);
A=[G1 G2; G4 G5];
B=[-G3; -G6]; %Gi for i=1:6 is function of T
X = A\B;
x=X(1); %function of T
y=X(2); %function of T
Eq= (x-b1x)^2+(y-b1y)^2 - d^2;
sol = solve(Eq,T,'returnconditions', true);
T_values = vpa(sol.T);
theta = 2*atan(T_values)
sol.conditions
vpa(sol.conditions)
We cannot execute because you have not defined the G variables.

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### Antworten (1)

Karan Singh am 9 Nov. 2023
Hi Mohamed,
The solve function in MATLAB is used to solve equations. By default, it returns all solutions without any conditions. However, in some cases, the solutions are only valid under certain conditions. The warning message you're seeing is MATLAB telling you that the solutions it found might only be valid under certain conditions. By specifying ReturnConditions, true, you're asking MATLAB to return these conditions along with the solutions.
However, the error message you're seeing is because you're trying to assign the output of solve with ReturnConditions, true to a single variable T_values. When you specify ReturnConditions, true,solve returns three outputs: the solutions, the parameters in the solutions, and the conditions on the parameters.
To fix the error, you should assign the output of solve to three variables, like this:
[sol, params, conditions] = solve(Eq, T, 'ReturnConditions', true);
After this, you can use vpa to convert sol to a numeric approximation:
T_values = vpa(sol);
Hope this helps!
Karan Singh Khati
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Walter Roberson am 9 Nov. 2023
When you specify “ReturnConditions”, “true”, “solve” returns three outputs:
syms p
sol = solve(p^2 > 3, p, 'returnconditions', true)
sol = struct with fields:
p: [2×1 sym] parameters: x conditions: [2×1 sym]
sol.p
ans =
sol.conditions
ans =
Single output is fine.
However
vpa(sol)
Unable to convert 'struct' to 'sym'.

Error in sym (line 273)

Error in vpa (line 46)
ss = sym(s);
That is because vpa() is not defined on a struct. You would need to vpa() something extracted from the struct
vpa(sol.p)
Karan Singh am 9 Nov. 2023
Thanks Walter for the correction!

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