∵a1+a5=6,∴a1=-1,
∴d=
| a5−a1 |
| 4 |
∴an=2n-3,Sn=n2−2n.
(2)∵an=2n-3,bn=2an,
∴bn=22n−3,
∴an•bn=(2n-3)•22n-3,
Tn=−1•2−1+1•21+3•23+5•25+…+(2n-3)•22n-3,
4Tn=-1×21+1•23+3•25+…+(2n-5)•22n-3+(2n-3)•22n-1,
两式相减,得:-3Tn=-
| 1 |
| 2 |
=-
| 1 |
| 2 |
| 2(1−22(n−1)) |
| 1−22 |
=
| (11−6n)•22n−11 |
| 6 |
Tn=
| (6n−11)•22n+11 |
| 18 |
