求数列1,1+2,1+2+22,1+2+22+23,…的前n项和.
解:∵,
∴
。
I don't know ______ with these books.[ ]
A. how I should do
B. what I should do
C. what should I do
D. how should I do
法兰密封面一般都要经过比较精密的加工,以保证足够的精度和光洁度。()