若|x+y+1|与$\sqrt{x-y-2}$ 互为相反数，则（3x-y）³的值为(         )

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                                                                                                             苔
                                                                                                          清·袁枚
                                                                                             白日不到处，青春恰自来。
                                                                                             苔花如米小，亦学牡丹开。