1.已知等差数列的{An}前n项和为sn,且s5=40,s8=100,求a1和公差d?2.已知等差数列{An}中,a1=4,d=-2,求S12

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1.已知等差数列的{An}前n项和为sn,且s5=40,s8=100,求a1和公差d?2.已知等差数列{An}中,a1=4,d=-2,求S12
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1.已知等差数列的{An}前n项和为sn,且s5=40,s8=100,求a1和公差d?2.已知等差数列{An}中,a1=4,d=-2,求S12
1.已知等差数列的{An}前n项和为sn,且s5=40,s8=100,求a1和公差d?2.已知等差数列{An}中,a1=4,d=-2,求S12

1.已知等差数列的{An}前n项和为sn,且s5=40,s8=100,求a1和公差d?2.已知等差数列{An}中,a1=4,d=-2,求S12
s5=40
5a1+5*4d/2=40
2a1+4d=16
s8=100,
8a1+8*7d/2=100
2a1+7d=25
3d=9
d=3
a1=2
(2)S12=12a1+12*11d/2=48-132=-84

s5=5a3=40,a3=8
s8=4（a4+a5）=100，a4+a5=25
于是2a3+3d=25，得d=3
a1=a3-2d=8-6=2
2，，，
sn=na1+n（n-1）d/2，
所以s12=12*4-12*11=-84

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