设等差数列前n项和Sn 已知a3=12 S12>0 S13
来源:学生作业帮助网 编辑:六六作业网 时间:2024/12/23 21:47:24
设等差数列前n项和Sn已知a3=12S12>0S13设等差数列前n项和Sn已知a3=12S12>0S13设等差数列前n项和Sn已知a3=12S12>0S13S12=12a1+12×11d/2=12(a
设等差数列前n项和Sn 已知a3=12 S12>0 S13
设等差数列前n项和Sn 已知a3=12 S12>0 S13
设等差数列前n项和Sn 已知a3=12 S12>0 S13
S12=12a1+12×11d/2
=12(a3-2d)+66d
=12a3+42d
=42d+12×12
=42d+144>0
d>-24/7
S13=13a1+13×12d/2
=13(a3-2d)+78d
=13a3+52d
=52d+13×12
=52d+156
等差数列中,S13=13a7<0,即a7<0.
S12=12*(a1+a12)/2>0,即a1+a12>0
又由于a1+a12=a6+a7,a7<0,a6+a7>0,得a6>0
设该数列公差为d,a6=a3+3d>0,a7=a3+4d<0
a3=12 ,12+3d>0且12+4d<0,解得d∈(-4,-3)求采纳噗,我好像有个地方搞错了哪里错了呢a6+a7大于0,...
全部展开
等差数列中,S13=13a7<0,即a7<0.
S12=12*(a1+a12)/2>0,即a1+a12>0
又由于a1+a12=a6+a7,a7<0,a6+a7>0,得a6>0
设该数列公差为d,a6=a3+3d>0,a7=a3+4d<0
a3=12 ,12+3d>0且12+4d<0,解得d∈(-4,-3)
收起
设等差数列前n项和Sn 已知a3=12 S12>0 S13
设等差数列的前n项和为Sn,已知:A3=12,S12>0,S13
设等差数列{an}的前n项和sn,已知a3=12,s12>0.s13
设等差数列{an}的前n项和为Sn,已知a3=11,S15*S16
设等差数列an 的前n项和为sn已知a3=12 S12>0 S13
设等差数列{an}的前n项和为Sn,已知a3=12,S12>0,S13
设等差数列{an}的前n项和为Sn,已知a3=12,S12>0,S130,S13
设等差数列{an}的前n项的和为Sn,已知a3=12,S12>0,S13
设等差数列{an}的前n项和为Sn,已知a3=12,且S12>0,S13
设等差数列{An}的前n项和为Sn ,已知a3=12 S12>0 S13
设等差数列an的前n项和为sn已知a3=12 s12大于0 s13小于0
设等差数列{an}的前n项和为Sn,已知a3=12,S12>0,S13
设等差数列的前n项和为Sn,a3=12,S12>0,S13
高二数学必修五一章等差数列的一道题,设等差数列{an}的前n项和为Sn,已知a3=12,S12>0,S13
等差数列{an}的前n项和Sn已知a3=12,S12>0,S13
设等差数列{an}的前n项和为sn,已知a3=5,s3=9
设等差数列{an}的前n项和为Sn,已知a3=11,S15>0,S16
已知等差数列前n项和为Sn,若Sn=17,求a3+a5