Texas Instruments TI-89, TI-92

176 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 176 of 26

Steps

TI-89

Keystrokes

›

TI-92 Plus

Keystrokes Display

1. Display the

MODE dialog box.

For Graph mode, select

DIFF EQUATIONS.

2. Display and clear the Y= Editor.

Then define the 1st-order

differential equation:

y1'(t)=.001y1ù(100ìy1)

Press

to enter the ùshown above.

Do not use implied multiplication

between the variable and

parentheses. If you do, it is treated as

a function call.

Leave the initial condition

yi1

blank.

¥#

ƒ8¸

¸.001

Y1pc100

|Y1d¸

¥#

ƒ8¸

¸.001

Y1pc100

|Y1d¸

3. Display the

GRAPH FORMATS

dialog box. Then set Axes = ON,

Labels = ON, Solution Method =

RK, and Fields = SLPFLD.

Important: To graph one differential

equation,

Fields

must be set to

SLPFLD

FLDOFF

. If

Fields=DIRFLD

an error occurs when you graph.

¥Í

DDB2

DB1

DB1¸

¥F

DDB2

DB1

DB1¸

4. Display the Window Editor, and

set the Window variables as

shown to the right.

¥$

0D10D

.1D0D

·10D110

D10D·10

D120D

10D0D.001

D20

¥$

0D10D

.1D0D

·10D110

D10D·10

D120D

10D0D.00

1D20

5. Display the Graph screen.

Because you did not specify an initial

condition, only the slope field is

drawn (as specified by

Fields=SLPFLD

in the

GRAPH

FORMATS

dialog box).

¥% ¥%

Preview of Differential Equation Graphing

Graph the solution to the logistic 1st-order differential equation y' = .001yù(100ìy). Start

by drawing only the slope field. Then enter initial conditions in the Y= Editor and

interactively from the Graph screen.

Important: With y1'

selected, the

TI-89 / TI-92 Plus will graph

the y1 solution curve, not

the derivative y1'.