连接F,E把ABCD分成四块,根据面积相加的关系得到
2AB^2-ABxBE-ABxCD-(AB-BE)x(AB-DF)=AExDF 化简
(AE+BE)xDF=AB^2
and AB^2=AE^2-BE^2=(AE+BE)x(AE-BE) 带进去刚好