Syntax: ACOSH(X)
X = a numeric value greater than or equal to 1.
ACOSH returns the hyperbolic arc cosine of angle X. The result is the angle (in radians) whose hyperbolic cosine is X.
Examples:
ACOSH(5) = 2.2924317
ACOSH(F5) = 5.192926, where F5 = 90
ACOSH(-1) = Error - ACOSH, domain is x 
= 1
Excel function: ACOSH