[krnhseya]

Hello.
I am studying those topics and I am stuck with 2 questions at the moment.
1) I need to prove that a = 4R/sqrt(3). R is the radius of atom and a is the edge length of unit cell. I know that two farthest points make 4R and I tried to work my way up to answer by using a face with diagnal line. But I don't get sqrt(3). I think I am thinking it wrong but I have no idea where.

2) I need to find planar density for FCC(100) and FCC(111). I managed to find FCC(100) which is 1/(4(R^2)*sqrt(2)). But I can't get the 111.

Thank you!

[AWK]

THis is a simple geometry:
diagonal in body centered cube is 4R
Hence 3a² = (4R)²