回复：小学生数学题 17

来源: 于 2013-11-13 08:22:26 [旧帖] [给我悄悄话] 本文已被阅读：次

from geometry point:

let DP perpendicular to AB, where P lies on AB. Since triangle ABC is iso-right triangle, so DP=1/2 AB = 1/ AD (as AB=AD)
Therefore, for right-triangle ADP, <DAP=30. should be easy going forward.