题目

 My friends have never heard of his name, ________ have I. A. so      B. either       C. neither      D. no 答案：CD为等腰Rt△ABC斜边AB的中点，DM⊥DN，DM，DN分别交BC，CA于点E，F．（1）当∠MDN绕点D转动时，求证：DE=DF．（2）若AB=2，求四边形DECF的面积．