Below are \triangle ABC△ABCtriangle, A, B, C and \triangle DEF△DEFtriangle, D, E, F. We assume that AB=DEAB=DEA, B, equals, D, E, BC=EFBC=EFB, C, equals, E, F, and AC=DFAC=DFA, C, equals, D, F. Here is a rough outline of a proof that \triangle ABC\cong\triangle DEF△ABC≅△DEFtriangle, A, B, C, \cong, triangle, D, E, F: