因为AB=AD=DC所以∠CAD=∠ACD,因为∠CAD与∠ACB为对顶角,所以∠CAD=∠ACDS
所以∠ACD=∠ACB,而在等腰梯形ABCD中∠ABC=∠BCD,所以∠ABC=2∠ACB
又因为,AC⊥AB,所以∠ABC+∠ACB=90°
故∠ABC=60°