(1)
S12>0
(a1+a12)*12/2>0
a1+a12>0
a1+a1+11d>o
24+11d>0
d>-24/11
(2)
S11
设等差数列（An）的 前n项和为Sn已知A1=12,S12>0,S11
2个回答
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