问题
解答题
已知数列{an}满足a1=1,an=
(1)求a2,a3,a4的值; (2)求证:数列{an-2}是等比数列; (3)求an,并求{an}前n项和Sn. |
答案
(1)∵数列{an}满足a1=1,an=
an-1+1(n≥2),1 2
∴a2=
a1+1=1 2
,a3=3 2
a2+1=1 2
,a4=7 4
a3+1=1 2
.…(3分)15 8
(2)∵
=an-2 an-1-2
=
an-1-11 2 an-1-2
=
(an-1-2)1 2 an-1-2
,1 2
又a1-2=-1,
∴数列{an-2}是以-1为首项,
为公比的等比数列.…(7分)1 2
(注:文字叙述不全扣1分)
(3)由(2)得an-2=-1×(
)n-1,则an=2-(1 2
)n-1,…(9分)1 2
∴Sn=2n-[1+
+(1 2
)2+…+(1 2
)n-1]=2n-1 2
=2n-2+(1×[1-(
)n]1 2 1- 1 2
)n-1.…(12分)1 2