a1+a2+a3+a4=4a1+6d=26
a(n-3)+a(n-2)+a(n-1)+an=4an-6d=110
两项相加 4a1+6d+4an-6d=26+110
a1+an=34
Sn=a1+a2+a3+.+an=(a1+an)*n/2=187
n=187*2/(a1+an)=187*2/34=11
a1+a2+a3+a4=4a1+6d=26
a(n-3)+a(n-2)+a(n-1)+an=4an-6d=110
两项相加 4a1+6d+4an-6d=26+110
a1+an=34
Sn=a1+a2+a3+.+an=(a1+an)*n/2=187
n=187*2/(a1+an)=187*2/34=11