题目

Training for a marathon requires careful preparation and steady, gradual increases in the length of the runs. 16   , buy the best-fitting running shoes you can find. No one can say which brand will work best for you or feel best on your feet, so you have to rely on your experience and on the feel of each pair as you shop. When you have found shoes that seem right, walk in them for a few days to double-check the fit. 17   . As always, you should stretch(伸展) at least ten minutes before each run to prevent injuries. During the first week, do not think about distance, but run five （本题9分）两个全等的直角三角形ABC和DEF重叠在一起，其中∠A=60°，AC=1. 固定△ABC不动，将△DEF进行如下操作：  （1）如图1，△DEF沿线段AB向右平移（即D点在线段AB内移动），连结DC、CF、FB，四边形CDBF的形状在不断的变化，但它的面积不变化，请求出其面积. （2）如图2，当D点移到AB的中点时，请你猜想四边形CDBF的形状，并说明理由. （3）如图3，△DEF的D点固定在AB的中点，然后绕D点按顺时针方向旋转△DEF，使DF落在AB边上，此时F点恰好与B点重合，连结AE，请你求出sinAED的值.