bd = 4, 设ED=x, AEC-FDC相似,
FD/x = 4/(x+4), FD = 4x/(x+4)
BF = 4 - FD = 4(x+4-x)/(x+4)= 16/(x+4)
ABC面积=BFA面积+BFC面积=1/2 * BF*（x+4) = 8