arccos range and domain: [−1, 1] → [0, π]
arccos takes inputs in [−1, 1] and returns angles in [0, π]. That range covers every angle a triangle can have, which is why the Law of Cosines never suffers an ambiguous case while the Law of Sines does.
Related Inverse identities
Practise it
Reading a formula is not the same as recalling it under time pressure. The workspace drills Inverse questions in the quiz, and the triangle solver shows a worked solution for any triangle you type in.
Or browse all 45 identities in the full reference, or read the trigonometry guides for the method behind them.