If you have an expression like ‘a+b^2’ on the stack and you wish to compute its value where ‘b=3’, you can simply store 3 in ‘b’ and then press = to reevaluate the formula. This has the side-effect of leaving the stored value of 3 in ‘b’ for future operations.
The s l (calc-let) command evaluates a formula under a
temporary assignment of a variable. It stores the value on the
top of the stack into the specified variable, then evaluates the
second-to-top stack entry, then restores the original value (or lack of one)
in the variable. Thus after ' a+b^2 RET 3 s l b RET,
the stack will contain the formula ‘a + 9’. The subsequent command
5 s l a RET will replace this formula with the number 14.
The variables ‘a’ and ‘b’ are not permanently affected in any way
by these commands.
The value on the top of the stack may be an equation or assignment, or a vector of equations or assignments, in which case the default will be analogous to the case of s t RET. See Storing Variables.
Also, you can answer the variable-name prompt with an equation or assignment: s l b=3 RET is the same as storing 3 on the stack and typing s l b RET.
The a b (calc-substitute) command is another way to substitute
a variable with a value in a formula. It does an actual substitution
rather than temporarily assigning the variable and evaluating. For
example, letting ‘n=2’ in ‘f(n pi)’ with a b will
produce ‘f(2 pi)’, whereas s l would give ‘f(6.28)’
since the evaluation step will also evaluate pi.