∴ad=bc,ad∥bc,ob=od= 1/2bd,bd=2ob

∫M is the midpoint of AD, then DE= 1/2AD= 1/2BC, that is, DM/BC= 1/2.

German mark ∨ BC

∴△DNM∽△BCN

∴DN/BN=DM/BC= 1/2

Then (OD-ON)/(OB+ON)= 1/2.

(OB- 1)/(OB+ 1)= 1/2

OB+ 1=2(OB- 1)

OB+ 1=2OB-2

OB=3

Then BD = 2OB = 6.

2、∵△DNM∽△BCN

∴MN/CN=DM/BC= 1/2

Then S△DMN/S△DCN=MN/CN= 1/2.

S△DMN= 1/2S△DCN= 1

∫DN = OD-ON = 3- 1 = 2

∴DN/BD=2/6= 1/3

∴S△DCN/S△BDC=DN/BD= 1/3

Then S△BDC=3S△DCN=6.

∫S△ABD = S△BDC = 6

∴S quadrilateral ABNM=S△ABD-S△DNN=6- 1=5.