Arccosine

acos x     acos[x]

Where x is a numeric, returns the arccosine of x; that is, the value whose cosine is x. The result is in radians and lies between 0 and π. (The range is approximate due to rounding errors). Null is returned if the argument is not between -1 and 1.

q)acos -0.4                     / arccosine
1.982313