function "_ARCSEC()" is wrong?

[SMcNeill]

Save and you should be good to go. (If not, then in the QB64PE menu, click DEBUG and then PURGE C++ LIBRARIES. That should rebuild the c-side of things and correct the issue after you've saved the changes above.)

I did that, but I still get an error message.

You've fixed it!

Before, it was running and giving answers for you, when it shouldn't have.  Now, it errors out in the range of -1 to 1, which is where the second method is printing "NaN" -- Not a Number as a result. 

That ERROR message *is* the fix in this case.