已知数列{an}的前n项和Sn=1-2+3-4+……+(-1)^(n-1)n,则S17+S33+S50
人气:309 ℃ 时间:2020-05-12 01:54:49
解答
S17=1+(-2+3)+(-4+5)+(-6+7)+.+(-16+17)=1+1+1+...+1=9
同理S33=17
S50=(1-2)+(3-4)+(5-6)+(7-8).+(49-50)=-25
所以S17+S33+S50=9+17-25=1
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