ACOS returns the inverse trigonometric cosine of a number.

ACOS (`number`)

This
function returns the inverse trigonometric cosine of `number`,
which is the angle (in radians) whose cosine is `number`.
The angle returned is between 0 and PI.

`=ACOS(-1)` returns `3.14159265358979
(PI radians)`.

`=DEGREES(ACOS(0.5))` returns `60`.
The cosine of 60 degrees is 0.5.