数学题
1.设数列{an}是等差数列,且a2=-6,a8=6,前n项和为Sn,当Sn最大时,则n=___. 2.在等差数列{an}中,若a10=10,a19=100,Sn=0,则n=___. 3.已知等差数列{an},an=2n-1,则S1+S2+S3+S4+S5=___. 4.在等差数列{an}中,d不等于0,已知S100=100S10,则a100/a10=___.
1.设数列{an}是等差数列,且a2=-6,a8=6,前n项和为Sn,当Sn最大时,则n=_无穷__. 2.在等差数列{an}中,若a10=10,a19=100,Sn=0,则n=17. 3.已知等差数列{an},an=2n-1,则S1+S2+S3+S4+S5=_55_. 4.在等差数列{an}中,d不等于0,已知S100=100S10, 则a100/a10=199/19
答:由求和公式,得 10A1+10*(10-1)*d/2=100…①, 100A1+100*(100-1)*d/2=10…②, 解得A1=10.99,d=-0.22...详情>>
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