Calculator User Manual

198 Chapter 11: Differential Equation Graphing
11DIFFEQ.DOC TI-89/TI-92 Plus: Differential Equation (English) Susan Gullord Revised: 02/23/01 11:04 AM Printed: 02/23/01 2:15 PM Page 198 of 26
In the
Y= Editor
Enter a valid system of two 1st-order equations. For
information about defining a valid system for a 2nd-
order equation, refer to page 186.
Set
Axes = CUSTOM:
TI
-
89
:
2‰
TI
-
92 Plus:
‰
If
Axes = TIME
, an
Invalid
axes
error occurs when
you graph.
If you enter initial conditions in the Y= Editor, the
equations referenced by the custom axes must have
the same number of initial conditions.
Otherwise, a
Dimension
error
occurs when you
graph.
With custom
axes
Set axes that are valid for your system of equations.
Do not select
t
for either axis. Otherwise, an
Invalid
axes
error occurs when you graph.
The two axes must refer to different equations in
your system of equations. For example,
y1
vs.
y2
is
valid, but
y1
vs.
y1'
gives an
Invalid axes
error.
In the Graph
screen
If the direction field is drawn but no curve is plotted,
enter initial conditions in the Y= Editor or select one
interactively from the Graph screen as described
starting on page 184. If you did enter initial
conditions, select
ZoomFit:
TI
-
89:
„j
A
TI
-
92 Plus:
„
A
The
ncurves
Window variable is ignored with
DIRFLD
.
Default curves are not drawn automatically.
Notes With
DIRFLD
, the equations referenced by the custom
axes determine which equations are graphed,
regardless of which equations are selected in the
Y= Editor.
If your system of equations refers to
t
, the direction
field (not the plotted curves) is drawn with respect to
one particular time, which is set by the
dtime
Window
variable.
Fields=DIRFLD