已知等差数列{an}的公差d>0,其前n项和为sn,满足S90,则n=多少时,sn最...答:Sn=n*(a1+an)/2 S9=9*(a1+a1+8d)/20 d>0,a1a1>-4.5d a1=-4d,an=-4d+(n-1)d=0, 得n=5 a1=-4.5d,an=-4.5d+(n-1)d=0, 得n=5.5 可知5,3,2a5=a1+a9=a2+a8=a3+a7=...S9=9a5<0 a5<0 a1+a10=a2+a9=a3+a8=a4+a7=a5+a6 S10=5(a5+a6)>0 ∵a5<0...