an=bn+c
Sn=(a1+an)*n/2=(b+c+bn+c)*n/2=n(b+bn+2c)/2有最小值
得b>0,{an}為等差增數(shù)列
∵a11/a10<-1,得a11>-a10>0>a10
11b+c>-10b-c>10b+c
21b+2c>0>10b+c,11b+c>21b+2c>0
當(dāng)Sn=n(b+bn+2c)/2>0,b+bn+2c>0
當(dāng)n=19時(shí),Sn=20b+2c<0,當(dāng)n=20時(shí),Sn=21b+2c>0,
∴得n最小為20