[Shah]

r(n)=(n^2)/Z * a0

a0=0.529A   

do these formulas work in all atoms?
Is there any relation between them?
what do we mean here by A?

[Jorriss]

A is an angstrom.

And no, it does not apply to all atoms. It only applies to hydrogen-like atoms. It is somewhat obvious with some thought it is unlikely to be valid for all atoms or else you would find many, many combinations of distinct atoms which have the same radii.

[Shah]

which radius are we calculating here? i mean s,p,or d .    when we say n = 2, or 3   what do we mean 2s or 2p... (3s or 3p .)

[NathanielZhu]

I'm guessing the s types since the guy said it only works on hydrogen-like atoms and s orbitals are the only ones that looks like a simple sphere.

The 2 in front of 2s, 2p, 2d, etc represent the energy level (how many electron shells you have) while s,p,d, etc represents the shape of the atom.