10.已知数列{an}为等比数列,其前n项和Sn=3n-1+t,则t的值为( )
| A. | -1 | B. | -3 | C. | $-\frac{1}{3}$ | D. | 1 |
9.等差数列{an}的前n项和为Sn,若a2+a4+a6=15,则S7的值是( )
0 233525 233533 233539 233543 233549 233551 233555 233561 233563 233569 233575 233579 233581 233585 233591 233593 233599 233603 233605 233609 233611 233615 233617 233619 233620 233621 233623 233624 233625 233627 233629 233633 233635 233639 233641 233645 233651 233653 233659 233663 233665 233669 233675 233681 233683 233689 233693 233695 233701 233705 233711 233719 266669
| A. | 28 | B. | 35 | C. | 42 | D. | 7 |