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