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问题 填空题
若二项式（
x
2
-
2
x2
）n的展开式的第三项是常数项，则n=______．
答案

展开式的通项为Tr+1=Cnrxn-3r（-1）r22r-n

∵展开式中第3项为常数项

∴当r=2时，x的指数为0

即n-6=0

∴n=6

故答案为6

阅读理解


Some students who are from all around the world are talking about the way they go to school on School Bus Bar at http://tieba.baidu.com/.
In our country, most of the students go to school by school bus. For me, I go to school by school bus every day. It takes me about 30 minutes to get to school, because my home is far from the school. The school bus is not only beautiful but wide and strong. It makes me feel safe when I am in school bus.
Mary Smith from America
I think riding is one of the best ways to go to school because my school isn’t very far from my home. It takes me about 15 minutes. I like riding a bike and I think it’s good for my health. But nowadays, the number of the cars is becoming larger and larger and the crowded traffic will certainly bring about the risk of safety. So my parents hope that I can take the school bus to school.
Li Ping from Wenzhou
I went to school by bus one month ago, but now I walk to school with my parents or my classmates. It’s because the school bus accident. In the accident, lots of students died. I have to get up one hour earlier than before. I spend one and a half hours getting to school. And now it’s very cold outside. I hope to have a school bus.
Bing Bing from Jiangsu
小题1:Mary Smith’s school bus isn’t ______.
A．safeB．beautifulC．strongD．dangerous
小题2:Why does Li Ping go to school by bike?
A．Because she likes riding.
B．Because the crowded traffic.
C．Because her school is far.
D．Because there is no school bus.
小题3:How long does Bing Bing spend on the way to school?
A．30 minutesB．15 minutes
C．one hourD．one and a half hours
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单项选择题

如图所示，在倾角为α的光滑斜面上置一刚性系数为k的弹簧，一质量为m的物体沿斜面下滑s距离与弹簧相碰，碰后弹簧与物块不分离并发生振动，则自由振动的固有圆频率为（）。

A.

B.

C.

D.
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