Derive density expression for FCC [100] in terms of the atomic radius r. 
Please format your solution. What are the numerical values of a and b? 
a) rho = (2 * a * b^3) / (a^3) 
b) rho = (3 * a * b^2) / (2 * a^3) 
c) rho = (4 * a * b^3) / (3 * a^3) 
d) rho = (b^3) / (2 * a^3)