Equilateral Triangle Inscribed In An Equilateral Triangle Small 320×315 Medium 640×631 Large 1024×1009 Download Tiff Original 2436×2400 | (262.4 KB) Description Illustration used to prove that triangle EFD is equilateral given that triangle ABC is equilateral and AE=BF=CD. Keywords congruent angles, equal, equilateral triangle, geometric proof, inscribed shape, triangles Galleries Geometric Theorems and Proofs Source: C. A. Hart & Daniel D. Feldman Plane And Solid Geometry (New York: American Book Company, 1911) 32