题目内容：

过梯形ABCD对角线交点作底的平行线分别交两腰AD,BC于点E,F.求证1/AB+1/CD=2/EF

优质解答

设对角线交点为O,有
OE/AB=OD/BD=OC/AC=OE/AB,故OE=OF
OE/AB+OF/CD=OD/BD+OB/BD=1,得
1/AB+1/CD=2/EF