数列-1,4,-7,10,…,(-1)n(3n-2)的前n项和为Sn,则S11+S20=()A.-16B.30C.28D.14-数学

题目简介

数列-1,4,-7,10,…,(-1)n(3n-2)的前n项和为Sn,则S11+S20=()A.-16B.30C.28D.14-数学

题目详情

数列-1,4,-7,10,…,(-1)n(3n-2)的前n项和为Sn,则S11+S20=(  )
A.-16B.30C.28D.14
题型:单选题难度:中档来源:不详

答案

S11=(-1+4)+(-7+10)+…-(3×11-2)=5×3-31=-16,
S20═(-1+4)+(-7+10)+…+[-(3×19-2)+(3×20-2)]=10×3=30,
所以S11+S20=14,
故选D.

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