Yes, ARCCOSINES is a Scrabble word!

Letter values in Scrabble US: A 1, R 1, C 3, C 3, O 1, S 1, I 1, N 1, E 1, S 1.

Is it a valid word?

ARCCOSINES is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

the inverse function of the cosine; the angle that has a cosine equal to a given number

Also: arc cosine, inverse cosine.

Not playable in Scrabble US: ARCCOS.