A:大宝
B:小宝
C:不能确定
如图,在四棱锥P﹣ABCD中,AB⊥PA,AB∥CD,且PB=BC=BD=6,CD=2AB=22,∠PAD=120°,E和F分别是棱CD和PC的中点.
求证:平面BEF⊥平面PCD
某同学验证物体质量一定时加速度与合力的关系,实验装置如图1所示.主要思路是,通过改变悬挂小钩码的质量,改变小车所受拉力,并测得小车的加速度.将每组数据在坐标纸上描点、画线,观察图线特点.
Mary ____.(like)
— What is ______ favorite color?
— Purple. Look! My watch is purple.
There are 30 classrooms ____________ our school. Mine is _____________ the second floor.