Problem with MVAR on HP42S

[GreyUser]

As mentioned, the error is because you’ve never created X in memory.

As written, your program requires that you store the value of interest into X and then execute COS. That’s not what you describe you wanted to do. Maybe you’re confused because MVAR is typically used with a solver programs or much larger, non-trivial programs with LOTS of variables.

I find it easiest to always create the variables used first before entering the program with:

STO
Shift
Alpha (enter key)
Then type out the name
EXIT when done

Then when you use MVAR or RCL (or anything that references a variable), you can just press the upper row key for that variable. If you have lots of them, you’ll have to use the up/down arrow keys to find the one of interest.

Take away: trivial programs don’t use/need MVAR. What are you actually trying to accomplish? I doubt it is to create a program to calculate a cosine...