This is not drawn to scale because the chords would be too
close together if it were.  Also nothing is lost by drawing 
the two chords parallel.



We draw in two radii, and a black line bisecting both
triangles into two right triangles. We label the
part of the part of the black line below the 8 as x.
We label the blue and red radii as r. 



We apply the Pythagorean theorem to the
two right triangles and we have this system
of two equations:









Since both left sides equal r2,









Substitute in













Ignore the negative value:



Edwin