∵AC⊥BC,AC=BC,∴△ABC是等腰直角三角形.
∵D为AB中点,CD⊥AB.又已知CD⊥DA1,
∵AB属于面ABB1A1,DA1属于面ABB1A1,∴CD⊥面ABB1A1,
∵BB1属于面ABB1A1,∴CD⊥BB1.又已知AB⊥BB1,
∵AB属于面ABC,CD属于面ABC,∴BB1⊥面ABC.