I'm trying to solve a quadratic equation by varying a variable multiple times

Hi, I'm very new to MATLAB so please forgive my question.

So I'm trying solve the quadratic equation to find yps by varying the value of pHs . We should expect to get 2 values of yps for each value of pHs.

This is the error matlab gives me. Subscripted assignment dimension mismatch.

pH =[5450  7000  9000 ];

for j=1:length(pHs)
    
ff(j)= yps/(1-yps) - alphas*(( xos - (pLs/pHs(j)) * yps) / ((1-xos)-(pLs/pHs(j))*(1-yps))) ;
yp(j)  = solve(ff(j),yps) ;    %error is in this line
ypp(j) = eval(yp(j)) ;

end

Answer 1

[Sulaymon Eshkabilov]

Hi,

Here is the corrected code:

% Since you are trying to solve your system symbolically then you'd need to
% assign symbolic variables with:
syms yps pLs alphas xos

% Or you can plug in the numerical values for:  [pLs, alphas, xos], then you
% will get the numerical solutions for ypp for each value of pHs. 
% Otherwise, you get now the solutions in a symbolic expressions in terms of: [alphas, xos, pLs].  

pHs =[5450  7000  9000 ];
for jj=1:length(pHs)  
ff= yps/(1-yps) - alphas*(( xos - (pLs/pHs(jj)) * yps) / ((1-xos)-(pLs/pHs(jj))*(1-yps))) ;
yp(:, jj)  = solve(ff,yps) ;    
ypp(:, jj) = eval(yp(:,jj)) ;
end

Note that if you have the numerical values of [pLs, alphas, xos], then you'd need to assign them before for loop or substitute in the equation.

Good luck.

[alfaisal albakri]

Thanks for your reply! Yes I assigned the symbolic variables but I didn't include it in my question so it doesn't look bloated .

Care to explain why the system gives a solution when you used

yp(:, jj)

instead of

yp(j)

I suspect it has something to do with the solution being a matrix

[Sulaymon Eshkabilov]

Hi Alfaisal,

Indeed, you have undersood correctly that in this case, your soultions have to be in a matrix form but not in a vector form. Good luck.