A:a5÷a5=0
B:a6×a4=a24
C:(x2﹣y2)2=x4﹣y4
D:(x3)3=x9
buses, restaurant, car, twenty, taxi
设a→、b→是不共线的两个向量,已知AB→=2a→+kb→,CB→=a→+3b→,CD→=2a→﹣b→,若A、B、D三点共线,求k的值.