证明：延长CB至E,使得BE=DN,
∵AB∥CD,则∠BAN=∠DNA,
在△ADN和△ABE中,
AD=AB
DN=BE
∠ADN=∠ABE,
∴△ADN≌△ABE,
∴∠BAE=∠DAN,
∵∠MAE=∠MAB+∠BAE,
∠BAN=∠BAM+∠MAN
∠MAN=∠DAN,∠DNA=∠E,
∴∠MAE=∠BAN,
∴∠DNA=∠MAE=∠E
即AM=ME,
∵ME=EB+BM
∴AM=EB+BM=DN+BM．
证完毕,