arctan𝑥
arctan3 asks the question "What angle in −𝜋2,𝜋2 makes 3 when you take the tangent?"
In other words, tan𝜃=3 has an infinite number of solutions, but arctan3 is only asking for the solution in −𝜋2,𝜋2 
Practice:
arctan1=
arctan0=