1.d=2 An=11 Sn=35 求n=?2.S9=-24 a1=-8 则d=?3.数列a,a,a,...的前n项和Sn=?4.在等差数列{An}中,a3=12,S12大于0,S13小于0,求公差d的取值范围.
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![1.d=2 An=11 Sn=35 求n=?2.S9=-24 a1=-8 则d=?3.数列a,a,a,...的前n项和Sn=?4.在等差数列{An}中,a3=12,S12大于0,S13小于0,求公差d的取值范围.](/uploads/image/z/3025367-71-7.jpg?t=1.d%3D2+An%3D11+Sn%3D35+%E6%B1%82n%3D%3F2.S9%3D-24+a1%3D-8+%E5%88%99d%3D%3F3.%E6%95%B0%E5%88%97a%2Ca%2Ca%2C...%E7%9A%84%E5%89%8Dn%E9%A1%B9%E5%92%8CSn%3D%3F4.%E5%9C%A8%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%EF%BD%9BAn%EF%BD%9D%E4%B8%AD%2Ca3%3D12%2CS12%E5%A4%A7%E4%BA%8E0%2CS13%E5%B0%8F%E4%BA%8E0%2C%E6%B1%82%E5%85%AC%E5%B7%AEd%E7%9A%84%E5%8F%96%E5%80%BC%E8%8C%83%E5%9B%B4.)
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1.d=2 An=11 Sn=35 求n=?2.S9=-24 a1=-8 则d=?3.数列a,a,a,...的前n项和Sn=?4.在等差数列{An}中,a3=12,S12大于0,S13小于0,求公差d的取值范围.
1.d=2 An=11 Sn=35 求n=?
2.S9=-24 a1=-8 则d=?
3.数列a,a,a,...的前n项和Sn=?
4.在等差数列{An}中,a3=12,S12大于0,S13小于0,求公差d的取值范围.
1.d=2 An=11 Sn=35 求n=?2.S9=-24 a1=-8 则d=?3.数列a,a,a,...的前n项和Sn=?4.在等差数列{An}中,a3=12,S12大于0,S13小于0,求公差d的取值范围.
1、∵Sn=na1+[dn(n-1)]/2 ∴a1=[35-n^2+n]/n 又∵an=a1+(n-1)d ∴a1=11-2(n-1)
所以可得n=5或7
2、∵Sn=na1+[dn(n-1)]/2 ∴S9=-24=9·(-8)+(d·9·8)/2 ∴d=4/3
3、∵a1=a,d=0 所以Sn=na1+[dn(n-1)]/2 =na
4、∵a3=a1+2d ∴a1=12-2d 又∵S12=na1+[dn(n-1)]/2 =12a1+66d=144+42d>0即d>-24/7
同理 S13=156+52d
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