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问题 选择题
已知函数f（n）=
n2，当n为奇数时
-n2，当n为偶数时
且an=f（n）+f（n+1），则a1+a2+a3+…+a100等于（　　）
A．0B．100C．-100D．10200
答案

∵an=f（n）+f（n+1）

∴由已知条件知，an=

n2-(n+1)2，n为奇数
-n2+(n+1)2，n为偶数

即an=

-(2n+1)，n为奇数
2n+1，n为偶数

∴an=（-1）n•（2n+1）

∴an+an+1=2（n是奇数）

∴a1+a2+a3+…+a100=（a1+a2）+（a3+a4）+…+（a99+a100）=2+2+2+…+2=100

故选B

单项选择题

疑为立克次体感染的患者，有诊断意义的实验室检查是

A．X线检查

B．变形杆菌OX19凝集试验

C．肥达反应

D．血液培养

E．检测血清中抗登革病毒的IgM抗体
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单项选择题

Home prices slid in November, raising questions about whether the housing recovery is robust enough to maintain a sustained turnaround. From October to November, home prices fell 0.2% after (1) 0.1% in October, according to a report Tuesday by Standard & Poor’s/Case-Shiller’s home price index. (2) five of the 20 metro areas (3) by the index saw price increases for the month.
On a(n) (4) basis, prices were 5.3% lower in November than in November 2008. Prices were the same as in late 2003. "What we’ve seen (5) the past couple of months is that the pace of (6) has fallen down," says Maureen Maitland of Standard & Poor’s. "Some markets have (7) Is that because we haven’t (8) the foreclosure (回赎权的取消) cycle Because of unemployment We’re not seeing the (9) we were seeing in the last summer months. "
Metro areas that have seen a retreat in home prices (10) Seattle, Charlotte, Las Vegas and Tampa. All four (11) gains they made in recent months (12) . (13) , for the 20-metro index, the annual rate of decline is improving. That (14) , with other recent housing reports that show prices (15) or accelerating, could point to (16) the housing market is starting to (17) its footing, some economists say. Existing home sale prices were $178 300 in December, which is 1.5% higher than December 2008, according to a report this week by the National Association of Realtors. That was the first year-over-year (18) in median price since August 2007.
"The most (19) thing we can say is not only the market is stabilizing, but we’ve seen that housing demand is p," says Bernard Baumohl, with the Economic Outlook Group. "Housing probably will continue to climb upwards through the summer, (20) it could slip a little as the tax credit expires. \

A．signs

B．index

C．news

D．marks
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