# Calculators TI-89 quick question

1. Sep 12, 2007

### milosport

I had a problem doing derivatives on my TI-89 and found the solution to my problem on this forum. I had x set to a constant instead of a variable. I'm just curious how x was set to this constant in the first place. I've never (that I know of) set it to anything. Just out of curiosity, what operations on the calculator would you want x to equal a constant and how is it done?

(edit: how is x set to a constant not how to do an operation with it)

Thanks,
Milo

2. Sep 12, 2007

### z-component

Using the STO (store) button at the bottom left, the syntax is "constant STO variable." This assigns a value to any variable you choose. That would explain how your derivative of x was zero. Check all your variables by selecting VAR-LINK (2nd + minus sign). Under the MAIN directory, you will see x is stored. Highlight this and remove it by pressing the left arrow (backspace) button and choose "yes."

I never store values into variables for this very reason. When I need to substitute a value for a variable in a complex expression, I use the | symbol to temporarily use a value for a certain variable. For example: 6x^2+7x+3|x=2 would return this expression as if x were '2.'

3. Sep 12, 2007

### milosport

Ah, great! I've never taken the time to really go over everything this calculator can do. I just figure things out as I need them. I wish I would have known about the "as if" thing a long time ago, could save some time. Although now that I think of it, using Y= and looking at the table seems easier. I love this calculator! Thanks for the help!

BTW, I'm a pilot too. Single engine land! Thanks again

Last edited: Sep 12, 2007