设⊙O与CD相切于点T连OT则OT⊥DC,AD‖DC,OM=OB
∴DT=CT=1/2DC=3 ,DC^2=DM×DA , 3^3=DM×6 ∴DN=3/2同理CT^2=CN×CA
CA=6√2,求得CN=3√2/4
过N作NF⊥DC于F,作NH⊥BC于H,
SMBCD=(3/2+6)×6×1/2=45/2
显见MHCF是正方形, NF/NC=1/√2∴NF=3/4
SMNFD=(3/2+3/4)(6-3/4)×1/2=189/32
SNHCF=NF^2=9/16
S△NBH=3/4×(6-3/4)×1/2=63/32
∴S△BMN=45/2-(189/32+9/16+63/32)=225/16=14.0625