Triangle ABC has been rotated 90° to create triangle DEF. Using the image below, prove that lines BC and EF have the opposite and reciprocal slopes. Triangles ABC and DEF are shown. A is at 4, 2. B is at 4, 5. C is at 1, 2. D is at 2, negative 4. E is at 5, negative 4. F is at 2,negative 1