distance Definition and 52 Threads

Even though I have all the formulas needed to do this problem, I cant figure out how to get rA, rB and rA' aswell as rB'

according to the figure, it's look like d is the distance between the center of two adjacent atoms, and so it should be simply L. I don't understand what d represents in the figure. that's the solution: Even if d is half the distance between two adjacent atoms that positioned diagonally (I...