设等差数列前n项和Sn 已知a3=12 S12>0 S13
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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,...
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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)
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