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数列{an}中,相邻两项an,an+1是方程x2+3nx+bn=0的两根,已知a...
数列{an}中,相邻两项an,an+1是方程x2+3nx+bn=0的两根,已知a10=-17,则b51的值等于( )
A.5800
B.5840
C.5860
D.6000
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B.5
C.6
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已知数列{a
n}满足:a
1=2,
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,若随机从数列{a
n}的前5项中选出两项相乘,则这两项之积等于12的概率为( )
A.
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B.
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C.
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D.
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等差数列{a
n}的前n项和为S
n,已知a
n-1+a
n+1-a
n2=0,S
2n-1=38,则n=( )
A.38
B.20
C.10
D.9
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△ABC中,BC=2,角B=
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,当△ABC的面积等于
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时,sinC=( )
A.
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B.
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C.
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D.
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我们把1,3,6,10,15,…这些数叫做三角形数,因为这些数目的点子可以排成一个正三角形(如图)
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则第七个三角形数是( )
A.27
B.28
C.29
D.30
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