Texas Instruments PLUS TI-89 - Manual (Page 202)

Chapter 11: Differential Equation Graphing 185
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 185 of 26
When a differential equation is graphed (regardless of whether a
solution curve is displayed), you can select a point on the Graph
screen and use it as an initial condition.
If Fields = Do this:
SLPFLD
– or –
DIRFLD
1. Press:
TI - 89 : 2 Š
TI - 92 Plus: Š
2. Specify an initial condition. Either:
¦ Move the cursor to the applicable point and
press ¸.
– or –
¦ For each of the two coordinates, type a value
and press ¸.
−
For SLPFLD (1st-order only), enter values
for t0 and y(t0) .
−
For DIRFLD (2nd-order or system of two 1st-
order equations only), enter values for both
y(t0) initial conditions, where t0 is the value
set in the Y= Editor or Window Editor.
A circle marks the initial condition and the solution
curve is drawn.
FLDOFF 1. Press:
TI - 89 : 2 Š
TI - 92 Plus: Š
You are prompted to select the axes for which you
want to enter initial conditions.
Your selections will be used as the axes for the
graph.
2. You can accept the defaults or change them. Then
press ¸.
3. Specify an initial condition as described for
SLPFLD or DIRFLD .
When you enter initial conditions in the Y= Editor or let ncurves
graph solution curves automatically, you can use … to trace the
curves.
However, you cannot trace a curve drawn by selecting an initial
condition interactively. These curves are drawn, not plotted.
Selecting an Initial
Condition
Interactively from
the Graph Screen
Note: With SLPFLD or
DIRFLD
, you can select
initial conditions interactively
regardless of whether you
enter initial conditions in the
Y= Editor.
Note: With FLDOFF, you
can select initial conditions
interactively. However, if
three or more equations are
entered, you must enter a
single value (not a list) as
the initial condition for each
equation in the Y= Editor.
Otherwise, a Dimension
error
occurs when graphing.
Note about Tracing
a Solution Curve
t is a valid selection.
It will let you specify
a value for t0.