Solution 11832: Correcting an "Undefined Variable" Error on the TI-89 Family, TI-92 Family, and Voyage™ 200 Graphing Calculators.
How can I correct an "Undefined variable" error on the TI-89 family, TI-92 family, and Voyage 200 graphing calculators?
An Undefined variable error occurs when the variable referenced is not defined.
For example, referencing a statistics variable when a statistical analysis has not been performed. If the variable is defined, then refer to the following instructions to resolve this error.
1) Press [2nd]  (MEM) [F1] [ENTER].
2) Select 2:Default.
3) Press [ENTER] and Defaults Set is displayed.
4) Press [ENTER].
Once the defaults have been reset, try the problem again.Please see the TI-89 family, TI-92 family and Voyage 200 guidebooks for additional information.