How to calculate radius ratio triangular voids?

The triangular voids are found in the planes of the close packed structures, whenever three spheres are in contact in such a fashion. Coordination number of triangular void is three. Here three atoms A, B, and C form triangular voids in which equilateral triangle form having angle 60⁰

Where, R= Radius of the (Atoms) sphere, r = maximum radius of a sphere that can be placed inside the void or radius of void.