∠ ade =∠ b +∠ bad (the outer angle of a triangle is equal to the sum of two non-adjacent inner angles)
∴∠EAD=∠ADE
∴EA=ED
∵EF⊥AD
∴EF is the center line on the side of AD.
2. extend AD to e and connect BE.
∴△BDE≌△ACD
∴∠E=∠CAD=90，BE=AC
In Rt△ABE ∠ bad = 30.
∴BE= 1/2AB
Namely: AC = 1/2ab.