【答案】解：设AD=x米，则AC=（x＋ 82）米．在Rt△ABC中，∵tan∠BCA=，∴AB=AC?tan∠BCA=2.5（x＋ 82）．在Rt△ABD中，∵tan∠BDA=，∴AB=AD?tan∠BDA=4x．∴2.5（x ＋82）=4x，解得x=．∴AB=4x=4×≈546.7．答：AB的长约为546.7米．
