【单选题】在一般工作条件下,齿面硬度HB≤350的闭式钢制齿轮传动,通常的主要失效形式为
【单选题】Tom asked my friend ____.
B.
that the earth is bigger than the moon
【简答题】在数列{a n }中, 且满足a n+1 -2a n +1=0 (1)求证数列{a n -1}是等比数列; (2)计算 .
【简答题】已知{a n }是公比为2的等比数列,若a 3 -a 1 =6,则a 1 =______; 1 a 21 + 1 a 22 +…+ 1 a 2n =______.
【单选题】Tom asked my friend_______.
B.
that the earth was bigger than the moon