A regular Polygon of an even number of sides being inscribed in a circle as ABC...A'...C'B'A' so that AA' is a diameter if two angular points next but one to each other as B, B' be joined, and the other lines parallel to BB' and joining pairs of angular points be drawn as CC', DD',...., then

(BB' + CC' +.... ) / AA' = A'B / BA

Similarly, in triangles KCG and GCL the CG = GC'
the angle G = 90 and are equal in both triangle because CC' is perpendicular to AA' the angle
C and C' are also equal due to two red line being parallel .