The `a S` (`calc-solve-for`

) [`solve`

] command rearranges
an equation to solve for a specific variable. An equation is an
expression of the form L = R. For example, the command `a S x`
will rearrange y = 3x + 6 to the form, x = y/3 - 2. If the
input is not an equation, it is treated like an equation of the
form X = 0.

This command also works for inequalities, as in y < 3x + 6.
Some inequalities cannot be solved where the analogous equation could
be; for example, solving @c{$a < b \, c$}
a < b c for b is impossible
without knowing the sign of c. In this case, `a S` will
produce the result @c{$b \mathbin{\hbox{\code{!=}}} a/c$}
b != a/c (using the not-equal-to operator)
to signify that the direction of the inequality is now unknown. The
inequality @c{$a \le b \, c$}
a <= b c is not even partially solved.
See section Declarations, for a way to tell Calc that the signs of the
variables in a formula are in fact known.

Two useful commands for working with the result of `a S` are
`a .` (see section Logical Operations), which converts x = y/3 - 2
to y/3 - 2, and `s l` (see section The Let Command) which evaluates
another formula with x set equal to y/3 - 2.

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