2. The tangent points of the smaller circles with the outer circle produce a regular pentagon. Let R denot the radius of the outer circle then the side of the pentagon is calculated as:
3. Use proportion in the indicated isosceles triangle:
4. The radius of the innermost circle, touching the smaller circle, is
5. This method can be used with all other examples where more than 2 circles are surrounded by one big circle. You only have to change the number of circles in the equation at 2.