16: Solving for x you should get

The problem now is that for a general x the expression arctan(tan(x)) does not give x but some x₀ from the basic interval of invertibility of tangent. If there is some integer k such that x − kπ is from this basic interval, then x₀ = x − kπ, the picture should explain it best.

In our case x − 2π works, so

Answer