Cesium Chloride Crystal Structure

[chemnerd]

I am working this problem...
http://www.fiu.edu/~saulj/Teaching/Physics2/01-Electric_Force/CrystalStructure.html
I got the answer to a as 0, but don't understand b.

 

[AspireBlack]

The quick way around part a is just argue that by symmetry all the forces sum to zero, since the attractions cancel pairwise. For part b, do exactly what they say and pull out one of the corner atoms. Some of the symmetries remain(3 pairs) but the remaining attraction is not canceled anymore... Plug and chug the rest into coulomb law. gl!