2007年广东省有一道中考题如下.如图,已知⊙O的直径AB垂直弦CD于点E,连结CO并延长交AD于点F,若CF⊥AD,AB=2,求CD的长.此题是6分题,属中档题型,题目简单,但阅卷时发现此题全市的平均得分才1.74分,空白率高达三分之一以上,几乎完全不会做此题的学生高达60%.只有10%的人得满分,得分率如此之低,出乎意料,这不得不引起我们对 Guangdong Province in 2007 there is a senior high school entrance examination questions are as follows. As shown, the diameter of the known ⊙ O AB vertical chord CD at point E, link CO and extend the intersection of AD and point F, if CF ⊥ AD, AB = 2, This question is a 6 sub-question, is a mid-range question type, the title is simple, but the marking found that the city's average score of 1.74 points, the blank rate as high as one-third or more, almost do not do this question students up to 60 %. Only 10% of people get the perfect score, scoring rate is so low, unexpectedly, which had caused us to