How do I set a parameter in a symbolic expression?

Dear Matlab Community,

I got a problem on how to set a parameter in my program. I solved some equations and the solution is a symbolic equation. This solution is correct. To keep it easy in this question: let the solution be like this:

syms a b
...%here are some equations that solve for a
...%this is the symbolic solution:
a = 3*b

Now I want to set b = 2 for example and wish to get the value for a. It should be a = 6 in that case. How can I set b = 2 and get the solution for a after getting the symbolic solution.

I tried:

a2 = solve(b == 2,a)

but it did not work. I hope someone could help me solving this maybe easy problem, then I can apply it to my original code which is a little more complicated. So I need to set b after solving symbolic for a.

Thank you! Stefan

Answer 1

[Mischa Kim]

Stefan, use

 syms a(b)
 a  = 3*b;
 a2 = subs(a,b,2)

[Mischa Kim]

Use the double command, see the example below.

 syms y(x)
 y  = x^(1/2)
 a1 = subs(y,x,2)
 a1 =
 2^(1/2)
 a2 = double(subs(y,x,2))
      1.4142

[Stefan Henning]

thanks a lot! this works fine

Answer 2

[Stefan Henning]

Thanks a lot! Now I get a solution. But one more thing: The value of my specific case has some form like this:

y = -1000/(300*3^(1/2)-100)

How do I get just one value and not an expression? In this case I would like to get

y = -2.383135547

Thank you!

[Mischa Kim]

Please add follow-up comments and questions as comments, not as answers. I have answered your question above.