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问题 选择题
已知等比数列{an}中，各项都是正数，且2a2，
1
2
a1，3a3成等差数列，则
a6+a7
a8a10
=（　　）
A．
4
27
B．2C．36D．12
答案

设等比数列的公比为q，则

∴2a2，

1
2
a1，3a3成等差数列，

∴a1=2a2+3a3

∴a1=2a1q+3a1q2

∴1=2q+3q2

∴q=-1或q=

1
3

∵等比数列{an}中，各项都是正数，

∴q=

1
3

∴

a6+a7
a8a10
=
a6+a7
a9
=
1+q
q3
=36

故选C．

单项选择题

ExtremeProgramming（XP）isadisciplineofsoftwaredevelopmentwith①（）ofsimplicity，communication，feedbackandcourage．Successfulsoftwaredevelopmentisateameffort-notjustthedevelopmentteam，butthelargerteamconsistingofcustomer，managementanddevelopers．XPisasimpleprocessthatbringsthesepeopletogetherandhelpsthemtosucceedtogether．XPisaimedprimarilyatobject-orientedprojectsusingteamsofadozenorfewerprogrammersinonelocation．TheprinciplesofXPapplytoany②（）projectthatneedstodeliverqualitysoftwarerapidlyandflexibly．AnXPprojectneedsa③（）customertoprovideguidance．Customers，programmers，managers，areallworking④（）tobuildthesystemthat’sneeded．Customers-thosewhohavesoftwarethatneedstobedeveloped-willlearnsimple，effectiveWaysto⑤（）whattheyneed，tobesurethattheyaregettingwhattheyneed，andtosteertheprojecttosuccess．

⑤（）

A.tell

B.know

C.communicate

D. feedback
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单项选择题

房室交界的自律性兴奋的次数，约为

A．10次/分
B．25次/分
C．50次/分
D．75次/分
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