Is there an elegant solution?
I want to solve the equation
3arcsin(5/(2R)) + arcsin(6/(2R)) + arcsin(8/(2R)) = pi
for R, where R is a positive real number. Of course R > 4 for the last term to be defined. I know that the solution is 5, and that this solution is unique [the derivative is negative, so the left side is monotonic]. However, I got this solution essentially from a numeric solver. Is there a nice way to find this solution?
I've thought of plugging both sides into the sine function, but then it turns into a long pseudo-polynomial problem, which doesn't seem helpful.
If it helps, this equation comes up in the following geometry problem. A pentagon is inscribed in a circle and has edge lengths 5, 5, 5, 6, and 8. What is the radius of the circle? An alternative computation of the radius is also acceptable.