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                                                                                                     渡荆门送别
                                                                                                           李白
                                                                                         渡远荆门外，来从楚国游。
                                                                                         山随平野尽，江入大荒流。
                                                                                         月下飞天镜，云生结海楼。
                                                                                         仍怜故乡水，万里送行舟。

若b＜0，则二次函数y=x²+bx-1的图象的顶点在（　　）