When triangle ABC is reflected across the y-axis to produce triangle DEF, the segments in triangle DEF will correspond to the segments in triangle ABC based on their relative positions to the axis of reflection.
For a reflection across the y-axis:
- If point B is at (x, y), then its reflection, point D, will be at (-x, y).
- If point C is at (x, y), then its reflection, point E, will be at (-x, y).
Thus, segment BC in triangle ABC corresponds to segment DE in triangle DEF.
Therefore, the segment on DEF that is congruent to segment BC is:
segment DE.