已知{an}是等差数列,其前n项和为Sn,a4=2,S5=20.设Tn=|a1|+|a2|+…+|an|,求Tn
问题描述：
已知{an}是等差数列,其前n项和为Sn,a4=2,S5=20.设Tn=|a1|+|a2|+…+|an|,求Tn
答
设公差为d ,则：a1 = 2 - 3d a2 = 2 - 2d a3 = 2 - d a4 = 2 a5 = 2+d ∵S5 = 20 ∴ 10 - 5d = 20 d = -2 ∴an = 10 - 2n以上希望对你有所帮助