a(n)=a+(n-1)d,
a(1)=a>0,
若d>=0,则a(n)>0,a(2007)*a(2008)>0,与a(2007)*a(2008)
an是等差数列,首项a1>0 a2007+a2008>0 a2007×a20080 S4015
an是等差数列,首项a1>0 a2007+a2008>0 a2007×a20080 S4015
数学人气：708 ℃时间：2019-11-02 18:14:14
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