It is usually [Shift] [cos]
though on some calculators it may be [2ndF] [cos]
The arc cos function is often written as cos-1 or acos, and on most calculators is written above the button to press; the colour of the text used to write it usually matches up with the key needed to press before the cos button.
On Casio calculators, it is [shift][cos] with the shift key labelled in yellow, and cos-1 written in yellow.
On a Texas Instruments calculator it is [2nd] [cos] with the 2nd key labelled in blue and the cos-1 written in blue.
On a Hewlett Packard calculator it is [<up_arrow_pointing_left>][cos] with the up_arrow_pointing left a white key with a black arrow and ACOS written in white above the cos button.
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