A:上游
B:中游
C:下游
D:中下游
若向量a→=(1,0,z)与向量b→=(2,1,2)的夹角的余弦值为23,则z=____ ,|a→-2b→|=____
如图所示,一偏心轮绕垂直纸面的轴O匀速转动,a和b是轮上质量相等的两个质点,则偏心轮转动过程中a、b两质点( )
下列操作正确的是( )
There ____ a full bin yesterday. But there is an empty one now.