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=-_数学解答

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

人气：207 ℃　时间：2020-04-13 18:18:10

解答

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