证明:延长AM至N,使MN=AM,并连接CN ∵BM=CM,∠AMB=∠NMC ∴△AMB全等于△NMC ∴AB=NC,∠ABM=∠NCM ∵四边形ABEF和四边形ACGH都是正方形 ∴AB=AF,AC=AH,∠BAF=∠CAH=90° ∴∠BAC+ ∠FAH=180° ∵∠ABC +∠ACB+ ∠BAC=180° ∴∠ACN=∠HAF ∵AB=AF=NC ∴△ANC全等于△HFA ∴AN=HF 即:FH=2AM