Exterior Angle of Triangle Theorem Small 320×253 Medium 640×505 Large 1024×809 Download TIFF Original 2400×1895 | (200.8 KB) Description Illustration used to prove that "If one side of a triangle is prolonged, the exterior angle formed is greater than either of the remote interior angles." Keywords exterior angle of a triangle, exterior angles, geometry, proof, prove, theorem, theorems, triangles Galleries Geometric Theorems and Proofs Source: C. A. Hart & Daniel D. Feldman Plane And Solid Geometry (New York: American Book Company, 1911) 52