求数列1,1+2,1+2+22,1+2+22+23,…的前n项和.
解:∵,
∴
。
—I don't see you at the meeting yesterday. Why?
—I ____ for a long distance call then from my daughter in Canada. [ ]
A. waited
B. am waiting
C. was waiting
D. will wait
风湿性关节炎晚期表现()。
A.自身免疫反应
B.病情活动
C.慢性,多发性,对称性,反复发作,四肢小关节
D.畸形
E.关节痛,肿,活动受限