Решение:

1. cos 6 = cos (2π - 6),

0 ≤ 2π -6 ≤ π, тогда

arccos(cos 6) = arccos(cos (2π - 6)) = 2π - 6;

2. sin 6 = sin (6 - 2π),

- π/2 ≤ 6 - 2π ≤ π/2, тогда

arcsin(sin 6) = arcsin(sin(6 - 2π)) = 6 - 2π.

3. arctg(tg 6) =

tg6 = tg(6 - 2π),

- π/2 < 6 - 2π < π/2, тогда

arctg(tg 6) =arctg(tg(6 - 2π)) = 6 - 2π.