undefined variable problem using vpasolve

So i need to solve a set of equations and wrote some code for that. Im not very confident in my matlab skills but i cant find the mistake in here. Im getting undefined variable error over and over again.

Heres the Code maybe someone here can help me.

% Parameter

A_1 = 8.23714;

A_2 = 8.19625;

B_1 = 1592.864;

B_2 = 1730.63;

C_1 = 226.184;

C_2 = 233.426;

c1 = 1.701;

c2 = 0.9425;

p = 1022.48; % [mbar]

T = 80; % [°C]

g1 = exp((c1.*((1-x1)).^2)./(((1-x1)+(c1/c2).*x1).^2));

g2 = exp((c2.*(x1).^2)./((x1+(c2/c1).*(1-x1)).^2));

p1 = 10^(A_1-(B_1./(T+C_1)));

p2 = 10^(A_2-(B_2./(T+C_2)));

syms x1

eqn = (g1.*p1.*x1)+(g2.*p2.*(1-x1))-p==0;

S = vpasolve(eqn,x1)

[Walter Roberson]

syms x1 before you define g1 since g1 and g2 use x1

Answer 1

[Shantanu Dixit]

Hi Vincent,

The error you are encountering is due to the fact that you are using the symbolic variable x1 in the calculations before declaring it as a symbolic variable using the syms function.

To resolve it, you need to declare x1 as a symbolic variable before using it in the equations.

% Parameter
A_1 = 8.23714;
A_2 = 8.19625;
B_1 = 1592.864;
B_2 = 1730.63;
C_1 = 226.184;
C_2 = 233.426;
c1 = 1.701;
c2 = 0.9425;
p = 1022.48;    % [mbar]
T = 80;         % [°C]

% Declare x1 as a symbolic variable
syms x1

% Calculate g1 and g2
g1 = exp((c1.*((1-x1)).^2)./(((1-x1)+(c1/c2).*x1).^2));
g2 = exp((c2.*(x1).^2)./((x1+(c2/c1).*(1-x1)).^2));

% Calculate p1 and p2
p1 = 10^(A_1-(B_1./(T+C_1)));
p2 = 10^(A_2-(B_2./(T+C_2)));

% Define the equation
eqn = (g1.*p1.*x1) + (g2.*p2.*(1-x1)) - p == 0;

% Solve the equation symbolically
S = vpasolve(eqn, x1)

[Vincent Nent]

Well it can be so easy. Thank you very much.