求证：△ABD≌△ACD．

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                                                                                              七律·长征
                                                                                           红军不怕远征难，
                                                                                           万水千山只等闲。
                                                                                           五岭逶迤腾细浪，
                                                                                           乌蒙磅礴走泥丸。
                                                                                           金沙水拍云崖暖，
                                                                                           大渡桥横铁索寒。
                                                                                           更喜岷山千里雪，
                                                                                           三军过后尽开颜。

若一个扇形的半径是18cm，且它的弧长是12π cm，则此扇形的圆心角等于（　　）