等差数列{an}中a1=50 d=-2则使前n项和sn为0,那么n=
1个回答
Sn=na1+n(n-1)d/2
=50n+(-2)n(n-1)/2
=51n-n²
=n(51-n)
令Sn=0
n(51-n)=0
n=0(n>0,舍去)或n=51
n=51
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