Symbolic Math Toolbox

The subs Command

You can substitute a numerical value for a symbolic variable using the subs command. For example, to substitute the value x = 2 in the symbolic expression,

• f = 2*x^2 - 3*x + 1
  

enter the command

• subs(f,2)
  

This returns f(2):

• ans =
       3
  

Note    To substitute a matrix A into the symbolic expression f, use the command polyvalm(sym2poly(f), A), which replaces all occurrences of x by A, and replaces the constant term of f with the constant times an identity matrix.

When your expression contains more than one variable, you can specify the variable for which you want to make the substitution. For example, to substitute the value x = 3 in the symbolic expression,

• syms x y
  f = x^2*y + 5*x*sqrt(y)
  

enter the command

• subs(f, x, 3)
  

This returns

• ans = 
  9*y+15*y^(1/2)
  

On the other hand, to substitute y = 3, enter

• subs(f, y, 3)
  ans = 
  3*x^2+5*x*3^(1/2)
  

The findsym Command

The Default Symbolic Variable