Sum of Interior Angles of a Polygon Proof Small 320×245 Medium 640×489 Large 1024×783 Download TIFF Original 2400×1834 | (297.0 KB) Description Illustration used to prove "The sum of all the angles of any polygon is twice as many right angles as the polygon has sides, less 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)