How can I give a number to a symbolic variable in MATLAB?

Question

I try to declare a symbolic variable in MATLAB with this code:

 syms a;

I do my computation on this variable and all thing is fine. But my problem is that, I don't know how can I give a number to variable and get the result as a number. For example the answer of my code is

   answer=exp(-10*a);

For instance I want to give 40 to "a" and get the answer as

   answer = 
        1.9152e-174

I really appreciate any suggestion.

Answer 1

You can use the SUBS function from the Symbolic Math Toolbox to perform symbolic substitution.

syms a;
answer = exp(-10*a);

subs(answer,a,40)


ans =

  1.9152e-174

Answer 2

use eval

syms a;
answer = exp(-10*a);

a=40;
eval(answer)

ans =
  1.9152e-174