4: You get
Now you are probably tempted to pull x2 out of the root, traditionally you would ignore the absolute value. Unfortunately, when you check, it turns out that this did not yield the right answer. The crucial moment in the check comes after you differentiate the arccosine and try to simplify it, you need to pull out something from under a square root and signs start to act up. It turns out that a mistake was made early in the solution, when you were trying to take the root of a tangent. After little experimentation you surely come up with an answer that works for both positive and negative x.