直角△ABC中，∠BAC =90°,D、E、F分别为AB、BC、AC的中点，已知DF=3，则AE= ____ .

动物的生殖方式包括两种。（  ）

阅读下面两段文字，按要求回答问题。
                                                                                             乡 愁
                                                                                           小时候
                                                                                           乡愁是一枚小小的邮票
                                                                                           我在这头
                                                                                           母亲在那头
                                                                                           长大后
                                                                                          乡愁是一张窄窄的船票
                                                                                          我在这头
                                                                                          新娘在那头
                                                                                          后来啊
                                                                                          乡愁是一方矮矮的坟墓
                                                                                          我在外头
                                                                                          母亲在里头
                                                                                          而现在
                                                                                         乡愁是一湾浅浅的海峡
                                                                                         我在这头
                                                                                        大陆在那头