Sum of Exterior Angles of a Polygon Proof Small 320×154 Medium 640×309 Large 1024×494 Download Tiff Original 2400×1157 | (146.1 KB) Description Illustration used to prove "If the sides of any polygon are prolonged in succession one way, no two adjacent sides being prolonged through the same vertex, the sum of the exterior angles thus formed is four right angles." Keywords angle, angles, angles of a polygon, exterior, geometry, interior, polygonal angles, proof, sum, theorem Galleries Geometric Theorems and Proofs Source: C.A. Hart, Daniel D. Feldman Plane and Solid Geometry (New York, NY: American Book Company, 1911) Courtesy the private collection of Roy Winkelman