Texas Instruments TI-89 Manual Book page 215

Fields=DIRFLD
198 Chapter 11: Differential Equation Graphing
In the Enter a valid system of two 1st-order equations. For
Y= Editor information about defining a valid system for a 2nd-
order equation, refer to page 186.
Set
Axes = CUSTOM:
: 2 ‰
TI-89
TI-92 Plus: ‰
If
Axes = TIME
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
error occurs when you
graph.
With custom Set axes that are valid for your system of equations.
axes
Do not select
error occurs when you graph.
axes
The two axes must refer to different equations in
your system of equations. For example,
valid, but
In the Graph If the direction field is drawn but no curve is plotted,
screen 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
„ j
TI-89:
TI-92 Plus:
The ncurves
Default curves are not drawn automatically.
Notes With DIRFLD
axes determine which equations are graphed,
regardless of which equations are selected in the
Y= Editor.
If your system of equations refers to
field (not the plotted curves) is drawn with respect to
one particular time, which is set by the
variable.
, an
Invalid
Dimension
t for either axis. Otherwise, an
y1 vs. y1' gives an Invalid axes
ZoomFit:
A
„
A
Window variable is ignored with
, the equations referenced by the custom
Invalid
vs. is
y1 y2
error.
DIRFLD .
t , the direction
dtime Window