晚上小东面向北极星站立,他的后面是____,左面是____.
Sorry, I can't ________ you. Can you ________ it again?
用数学归纳法证明“1+a+a2+…+an+1=1-an+21-aa≠1,n∈N*”时,在验证n=1成立时,左边应该是( )