Taylor expanding with the HP50G

[bxparks]

I attempted to use the CAS features of the 50g for the first time yesterday. I tried to do a relatively simple Taylor series expansion using the TAYLOR0 and TAYLR functions:

TAYLOR0(N*X/((1+X)^N-1)

and the equivalent

TAYLR(N*X/((1+X)^N-1), X, 5)

I got both error messages described in this thread:

- "Approx. Mode off? YES NO"
- "MODE switch not allowed here"

Not the most helpful error messages. I tried 10-20 times with different variations, but I could not get this to work.

I was about to throw the 50g out the window, but decided to do an internet search first. There are apparently 2 places on the planet that mention these error messages, and this is one of them. (Funny enough, the other one is the following thread from 2021 (HP49-50G : Taylor) where Gil posts basically the same question, and figures out basically the same answer.)

It turns out my problem is that 'N' was already defined. How is that possible? I don't even know how to create a variable on the 50g. (I've tried, that's another story.) It's because I had used the [LEFT] [FINANCE] app, which creates the TVM variables (N, I%YR, PV, PMT, FV), and N happens to be one of them.

The solution was to delete the `N` variable. But how do I do that? Since this is the HP-50g, it's completely non-trivial. I figured it out yesterday, but this morning, I could not remember. For the record, in case someone else (e.g. my future self) runs into this problem, here are the instructions for how to delete a variable:

- Go to [LEFT] [FILES] (not the VAR button as I had guessed)
- Press [ENTER] to select {Home} (I think. I have no idea what that "CASDIR" subdirectory is)
- Scroll down to the variable that you want to remove
- Press the [NXT] button
- Press the [PURGE] soft menu (F1) button (I would have expected it to be labeled "DEL", and I would have expected either the <- button or the [LEFT] [DEL] button to work)
- Press F1 to answer "YES" to the prompt