Proof: By Euclid
(related to Corollary: 6.20: Similar Polygons are Composed of Similar Triangles)
It follows from Prop 6.19 and the fact that every rectilinear figure can be triangulated.
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References
Bibliography
 Byrne, O.: "The First Six Books of the Elements of Euclid, in Which Coloured Diagrams and Symbols are used Instead of Letters", London William Pickering, 1847