X = a numeric value between -1 and 1.
ACOS returns the arc cosine of angle X. The result is the angle (in radians) whose cosine is X and ranges from 0 to . To convert the result from radians to degrees, use the DEGREES function.
ACOS(1) = 0
ACOS(R3) = 1.670963748, where R3 = -0.1
DEGREES(ACOS(0)) = 90
ACOS(-20) = Error - ACOS, domain is -1 = x = 1
Excel function: ACOS