|
∴当n≤13时,{an}的前n项和为Sn=
25n−n2 |
2 |
当n>13时,{an}的前n项和为Sn=
1 |
2 |
满足ak+ak+1+…+ak+19=102,即ak+ak+1+…+ak+19=Sk+19-Sk-1=102,k是正整数
而Sk+19=
1 |
2 |
1 |
2 |
①当k-1≤13时,Sk-1=-
1 |
2 |
27 |
2 |
所以Sk+19-Sk-1=
1 |
2 |
1 |
2 |
27 |
2 |
②当k-1>13时,Sk-1=
1 |
2 |
1 |
2 |
所以Sk+19-Sk-1=
1 |
2 |
1 |
2 |
综上所述,满足条件的k=2或5
故答案为:2或5