菱形AEDF=>AE//DF且AE=AE=DF=DE
由AE//DF可知△ABC相似于△FDC,则AB/DF=AC/CF,又DF=AF,AF+CF=AC=8,得
10/AF=8/(8-AF),计算得AF=40/9,即菱形AEDF的边长为40/9