设AM=a,DE=b,
DE=BC-AM,
DE=FG=MN=b
△ADE∽△ABC,
b/(a+b)=（a-b）/a,
a²-ab-b²=0
a=1/2(根号5+1),
AM:BC=a/(a+b)=1/2（根号5-1）=0.618
另可由b/(a+b)=（a-b）/a推出AM:BC为黄金分割比例=1/2（根号5-1）=0.618