Question

已知等差数列{an}满足a3*a7=-12,a4+a6=-4.当数列{an}的公差小于零时,求n取何值时,前n项和Sn有

Answer 1

等差数列{an} a4+a6=a3+a7=-4

a3*a7=-12

a3=2,a7=-6

或a3=-6,a7=2 数列{an}的公差小于零

所以 a3=2,a7=-6

a3=a1+2d=2

a7=a1+6d=-6 d=-1 a1=4

an=a1+(n-1)d=5-n

an>=0 n