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

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 section 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`

.

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