When you learn to ride a bike, you have to learn to _______ first.
A:keep your balance
B:keep your promise
C:keep calm
(Ⅰ)求证:A1B∥平面B1CD;
(Ⅱ)当三棱锥C﹣B1C1D体积最大时,求点B到平面B1CD的距离.
先化简,再求值:1a+1﹣a(a+1)2,其中a=2﹣1.