Hello i got no problem with domain and im a bit of confused on how to find the range of a function given:

$\displaystyle f(x) = \sqrt{4-x^2}$

the answer is $\displaystyle [0,2] $

cud u please explain how its done? i really dont understand how to find the range

Ive got another question:

Why is $\displaystyle x = y^2$ not a function and $\displaystyle x = y^3$ is?