ACOSH (Arccosine Hyperbolic inverse)

Purpose: 
Returns the inverse hyperbolic cosine of a number.

Command:
=ACOSH(number)

Example:
Enter value of 2 in cell A1. In cell A2, enter the following formula:
=ACOSH(A1)

This will return the value of 1.32

Please see image below:

ACOS (Arccosine)

Purpose: 
Returns the arccosine of a number, in radians in the range 0 to Pi. The arccosine is the angle whose cosine is number.

Command:
=ACOS(number)

Example:
Enter value of 0.5 in cell A1. In cell A2, enter the following formula:
=ACOS(A1)

This will return the value of 1.05

Please see image below: