∠CAD=∠DCA=∠ABD=∠CBD，∠ADB=∠ACB=90
∴△AED∽△BDA,DA/AB=AE/BD
BD again? =4? - 1?
∴AE=√ 15/4,AC=√ 15/2
BC again? =4? -AC?
∴BC=7/2