HP -28S Manual page 114

Normal calculator operation is suspended while FORM is active. The
FORM display shows the equation with all sub expressions delimited
by parentheses. You'll use FORM to manipulate sub expressions
within the equation.
The goal is to combine
(A
*
>()
and
(2
*
:x:)
in a single term
( ( A
+2 )
*::-:: ).
There are three steps required, shown below as you
might write them on paper. The current form of the equation is:
(ax
+
ab)
+
2x
=
c
The first step is to commute ax and ab, giving:
(ab
+
ax)
+
2x
=
c
The second step is to associate ax and 2x, giving:
ab
+
(ax
+
2x)
=
c
The third step is to merge ax and 2x, giving:
ab
+
(a
+
2) x
=
c
Step 1: Commute ax and abo
Move the cursor (the inverse character or characters) to
+.
The position of the cursor determines which subexpression you're act-
ing on. Here you want to act on the sub expression ( ( A
*
>0 + ( A
*
8) )
to commute the arguments to
+.
9: Symbolic Solutions 113