New tests have been added to the problem which exploit a common mistake when using acos function. Hint: try to calculate acos(sqrt(3.0) * sqrt(3.0) - 4.0) in your programming language. 269 authors (58%) with accepted solutions have been challenged.

Why is this a mistake?

How to modify the solution to avoid the mistake? Who knows?

OK, just need to add a clamp for acos' arg.