About ______ years ago, Yangliuqing New Year pictures began to enjoy great fame, with every family adept at creating this particular genre of painting.
【简答题】关于平面向量有下列四个命题: ①若 a ? b = a ? c ,则 b = c ,; ②已知 a =(k,3), b =(-2,6).若 a ∥ b ,则k=-1. ③非零向量 a 和 b ,满足| a |=| b |=| a - b |,则 a 与 a + b 的夹角为30°. ④( a | a | + b | b | )?( a | a | - b | b | )=0. 其中正确的命题为 _...