Tom can play the guitar, ________ he can't play it very well.
质量为2kg的物体,速度由4m/s变为﹣6m/s,则此过程中,它受到的合外力的冲量为( )
(Ⅰ)证明:直线EF∥平面ACD
(Ⅱ)证明:直线HG∥平面CEF.