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