A function f is defined for example by :
f(x):=x^
2-1 or by f:=x->x^
2-1
that is to say, for all x, f(x) is equal to the expression
x2−1. In that case, to have the value of f for x=2, input :f(2).
But if the input is
g:=x^
2-1, then g is a variable where the
expression x2−1 is stored. In that case, to have the value of g for x=2,
input : subst(g,x=2) (g is an expression depending on x).
When a command expects a function as argument, this argument should
be either the definition of the function (e.g. x->x^
2-1)
or a variable name assigned to a function (e.g. f
previously defined by e.g. f(x):=x^
2-1).
When a command expects an expression as argument, this argument should
be either the definition of the expression (for example x^
2-1),
or a variable name assigned to an expression (e.g.
g previously defined, for example, by
g:=x^
2-1), or the evaluation of a function. e.g.
f(x) if f is a previously defined function,
for example, by f(x):=x^
2-1).