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等差数列{an}中,a3=8,a7=20,若数列{}的前n项和为,则n的值为A.14B.1...
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等差数列{a
n}中,a
3=8,a
7=20,若数列{
}的前n项和为
,则n的值为( )
A.14
B.15
C.16
D.18
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