6. Make an external bisector, connect AB and make the middle vertical line of AB intersect at one point, which is what you want.

7. connect DC DB

Ed divided BC vertically.

∴DB=DC

That is to say, the ad is divided equally ∠BACDM⊥AB,DN⊥AC.

∴DM=PN∠DMB=∠DNC=90

∴Rt△DMB≌Rt△DNC(HL)

∴BM=CN