HP 40gs - Manual (Page 186)

14-4 Computer Algebra System (CAS)
10.Press three times to
select the entire
expression and then
press
to simplify
it to the form required.
CAS variables
When you use the symbolic calculation functions, you are
working with symbolic variables (variables that do not
contain a permanent value). In the HOME screen, a
variable of this kind must have a name like S1…S5,
s1…s5, n1…n5, but not X, which is assigned to a real
value. (By default, X is assigned to 0). To store symbolic
expressions, you must use the variables E0, E1…E9.
In the Equation Writer, all the variables may, or may not
be, assigned. For example, X is not assigned to a real
value by default, so computing X + X will return 2X.
Moreover, Equation Writer variables can have long
names, like XY or ABC, unlike in HOME where implied
multiplication is assumed. (For example ABC is
interpreted as A × B × C in HOME.) For these reasons,
variables used in the Equation Writer cannot be used in
HOME, and vice versa.
Using the PUSH command, you can transfer expressions
from the HOME screen history to CAS history (see
page 14-8). Likewise, you can use the
POP command to
transfer expressions from CAS history to the HOME
screen history (see page 14-8).
The current variable
In the Equation Writer, the current variable is the name of
the symbolic variable contained in VX. It is almost always
X. (The current variable is always S1 in HOME.)
Some CAS functions depend on a current variable; for
example, the function DERVX calculates the derivative
with respect to the current variable. Hence in the Equation
Writer, DERVX(2*X+Y) returns 2 if VX = X, but 1 if VX
= Y. However, in the HOME screen, DERVX(2*S1+S2)
returns 2, but DERIV(2*S1+S2,S2) returns 1.
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