ACOS function
This article describes the formula syntax and usage of the ACOS function in Microsoft Excel.
Description
Returns the arccosine, or inverse cosine, of a number. The arccosine is the angle whose cosine is number. The returned angle is given in radians in the range 0 (zero) to pi.
Syntax
ACOS(number)
The ACOS function syntax has the following arguments:

Number Required. The cosine of the angle you want and must be from 1 to 1.
Remark
If you want to convert the result from radians to degrees, multiply it by 180/PI() or use the DEGREES function.
Example
Copy the example data in the following table, and paste it in cell A1 of a new Excel worksheet. For formulas to show results, select them, press F2, and then press Enter. If you need to, you can adjust the column widths to see all the data.
Formula 
Description 
Result 
=ACOS(0.5) 
Arccosine of 0.5 in radians, 2*pi/3 
2.094395102 
=ACOS(0.5)*180/PI() 
Arccosine of 0.5 in degrees 
120 
=DEGREES(ACOS(0.5)) 
Arccosine of 0.5 in degrees 
120 