Question

等差数列｛a_n｝的前n项和为S_n,若S_16>0,S_17

Answer 1

Sn=na1+n(n-1)/2)d=(d/2)n^2+(a1-d/2)n

n=-b/2a=-(a1-d/2)/2(d/2)=1/2-a1/d>1/2+15/2=8

S16=16a1+120d>0 16a1>120d 16a1/d