***First part solved skip to post #11 ^^^^^^^^^^^^^^^^^^^^^^^^^^^ It's a silly title, but I really need help. The four pink balls surround the purple one and all make contact. Calling the radius of the purple ball r and the radii of the pink R: Show that r/R=.225. 3. The attempt at a solution I think that this can be done with simple plane trig, but I am having troubles with the visualization. Anybody have any ideas? I am dying here. edit: I hijacked this image to simplify the problem. The original problem statement had no image and came from a Materials Science text: "Show that the minimum cation-to-anion ratio for a coordination number 4 is .225"