设 a , b , c 是任意的平面向量,给出下列命题: ① ( a ? b ) c =( b ? c ) a ②若 a ? b = a ? c ,则 a ⊥( b - c ) ③ ( a + b )( a - b )=| a | 2 -| b | 2 ④ ( a ? b ) 2 = a 2 ? b 2 其中正确的是______.(写出正确判断的序号)
【简答题】Task 2: listen and choose the best answer for each question. 1. When does the caller want to stay in the hotel? A. on Tuesday 14 B. on Tuesday 23 C. on Thursday 23 2. Which luggage does ...