∵等边△abc∴AB=AC ∠BAC=∠ACB 又因为AD=CE ∴△ABD≌△CAE∴∠CAE=∠ABD又∠CAE+∠BAE=60°∴∠ABD+∠BAE=60°∴∠ANB=120°∴∠BNM=60°又BM⊥AE∴∠MBN=30°∴MN=½BN