How to substitute the values of two variables in an expression at the same time - Matlab

Hello,

I have the following expression:

Q1sym =
 
6562864383290039/3377699720527872 - (1321331071190089*(-(10*(b/5 - 1)*((2*b)/5 + b*(2*b + 1) - t*(b/5 - 1) + 2))/3)^(3/10))/1125899906842624
 

for which I want to substitute at the same time the two unknowns (b,t) for which I have the following 5 values:

b1 =
 
[ 19/12 - 457^(1/2)/12, 317/200 - 1138401^(1/2)/600, 119/75 - 283576^(1/2)/300, 953/600 - 1130209^(1/2)/600, 159/100 - 281529^(1/2)/300]

t =
 
[ 1/2, 11/20, 3/5, 13/20, 7/10]

and I am using the following code:

Q1 = subs(Q1sym, [b t] ,[b1 t])

but it doesn't work.

Do you know how I can solve that?

Thanks a lot

Answer 1

[Paul]

Can you define Q1sym as a symfun?

% simple example
syms b t
Q1sym(b,t) = b + t;
b1 = sym(1:5);
t1 = sym([ 1/2, 11/20, 3/5, 13/20, 7/10]);
Q1sym(b1,t1)

ans = 

[Myrto Kasioumi]

Thank you that worked!!

Answer 2

[Walter Roberson]

Q1 = subs(Q1sym, {b, sym('t')}, {b1, t});

Notice you had to create a local symbolic variable named 't' to pass in, to be a reference for the t variable that is no longer a simple symbolic variable.

I recommend that you get in the practice of avoiding writing over any symbolic variable that there is still any reference to. In this case, there was still a reference to t at the time you overwrote the MATLAB variable t with the vector. If you had named the vector of replacement values something like t1 (in analogy with b1) then you could have done

Q1 = subs(Q1sym, {b, t}, {b1, t1});

[Myrto Kasioumi]

I understand. Thanks for the clarification and the help.