{an}是等差数列,a2=-1,a8=5,则数列{an}的前9项和S9=______.
∵a2+a8=2a5=4
∴a5=2
∴S9=(a1+a9)×92=2a5×92=18
If you live in a big city, you're surrounded by noise from cars, buses and trains. [ ]
A. briefly
B. instantly
C. constantly
D. awfully
舟状头畸形的颅缝闭合特点一般表现为
A.矢状缝、人字缝提早闭合B.矢状缝、冠状缝提早闭合C.冠状缝、人字缝提早闭合D.矢状缝、顶颞缝提早闭合E.偏侧颅缝提早闭合