ARCCOS(1) and ARCCOS(1) are not calculated in Excel
2 comments

Harlan Grove commented
The Excel team has never done a good job with boundary cases for special numeric functions. For a long time, GAMMALN(1) and GAMMALN(2) didn't return 0. I figure the Excel team figures those who really need precision for such things would be using MATLAB or other math packages rather than Excel. Excel really isn't a general mathematical tool.

Wiens commented
I solved it using ACOS(MIN(1; SOME_FORMULA)) or ACOS(MAX(1; SOME_FORMULA))
ACOS(1) and ACOS(1) are not calculated in Excel. It gives an error.
I don't want to use a rounded number almost equal to 1 or 1. It gives a calculation error.