1-1 probably means "one-to-one", a term you ought to be familiar with.

x = 0 is OK for the domain! arctan(0) = 0 and all is well. The domain is all real x.

The range is (-pi/2, pi/2). That's easy to see if you consider the domain of the inverse of t(x), that is, (1/2) tan (x) ....