Texas Instruments TI-89 Manual Book page 101

Combining Multiple
Units
Tip: Create a user-defined
unit (page 88) for frequently
used combinations.
Using Parentheses
with Units in a
Calculation
Tip: If you have any doubt
about how a value and its
units will be evaluated,
group them within
parentheses ( ).
84 Chapter 4: Constants and Measurement Units
You may need to combine two or more units from different
categories.
For example, suppose you want
to enter a velocity in meters per
second. In the dialog
UNITS
box, however, the
Velocity
category does not contain this
unit.
You can enter meters per
second by combining
_m
_s from the Length and
categories, respectively.
In a calculation, you may need to use parentheses ( ) to group a value
and its units so that they are evaluated properly. This is particularly
true for division problems. For example:
To calculate: Enter:
100_m
100_m/(2_s)
2_s
If you omit the parentheses, you will get
unexpected units. For example:
100_m/2_s
Here's why you get unexpected units if you do not use parentheses.
In a calculation, a unit is treated similar to a variable. For example:
is treated as
100_m
and
is treated as
2_s 2ù _s
Without parentheses, the entry is calculated as:
100ù _m
100ù _m / 2ù _s =
3ù 9.8_m/_s
and
Time
50.ø
You must use parentheses for (2_s).
This is important for division.
50.ø_mø_s
100ù _m
ø ø
ù _s = 50.
_m
2
Combine the units _m
and _s. There is no pre-
defined _m/_s unit.
_m
_s
_s