做AE垂直BC于E.则BE=CE.AD^2=AE^2+DE^2 (1) AC^2=AE^2+CE^2 (2)
(2) - (1) = AC^2 - AD^2 = CE ^2 - DE^2 = AB^2 - AD^2 = (CE+DE) x (CE-DE)
=CD x (BE-DE)=CDxBD 所以 AB^2 = AD^2 + BD x DC. 根据已知条件求出AB即可.
