The acosh function returns the inverse hyperbolic cosine. Therefore,
ACOSH( COSH( z ) ) = z
The equation used to calculate the inverse hyperbolic cosine is:Further information on the inverse hyperbolic functions can be found on Wikipedia
The Excel ACOSH function calculates the inverse hyperbolic cosine of a supplied number.
The format of the function is :
Where the number argument is a positive real number, which must be greater than or equal to 1, and which you want to calculate the inverse hyperbolic cosine of.
The number argument can be input to the function either :
  directly 
  as a value returned from another function or formula, or 
  as a reference to a cell containing a value 
Column B of the spreadsheet below, shows examples of the Excel Acosh Function, used to calculate the inverse hyperbolic cosine of three different values.
Formulas:

Results:

Further information and examples of the Excel Acosh function can be found on the Microsoft Office website.
If you get an error from the Excel Acosh Function, this is likely to be one of the following:
#NUM!    Occurs if the supplied number argument is less than 1 
#VALUE!    Occurs if the supplied number argument is nonnumeric 