Question

17、已知等差数列{an}中,a3a7=-16,a4+a6=0,求{an}前n项和Sn.

Answer 1

a4+a6=0＝a3+a7

又a3a7=-16

∴a3=-4,a7=4,此时an＝2n－10,Sn＝n^2-9n

或a3=4,a7=-4,此时an=-2n＋10,Sn＝-n^2+9n