Hi, aleschio.
I don't think we need to know I will use to denote the measure of a length. My idea is something like this:
1) Since the base is equilateral we know (see Equilateral triangle - Wikipedia, the free encyclopedia). There are many ways to show that (for example see Viviani's Theorem Viviani's theorem - Wikipedia, the free encyclopedia). Hence
2) Since the tangents to the circle DF and DA are drawn from the same point (i.e. D), we know Therefore, and Since we also know that
3) We now use the right triangle and the tangent function to obtain the radius.
Does this answer your question? Let me know if anything is unclear/incorrect.
Good luck!
Thanks GJA
I have a problem that I have to count the ratio of this pyramid to sphere inscribed in it when and are given. I wondered if it's possible to count it when only is known. Your solution says that it is possible and fairly easy. But I have one more question, Is obvious that sphere is tangent to the base in point which is the end of the height of this pyramid or do we have to proof it?