问题
解答题
设数列{an}满足a1=0,且an+1=an+
(Ⅰ)求a2的值; (Ⅱ)设
(Ⅲ)设g(n)=
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答案
(Ⅰ)∵a1=0,且an+1=an+
+1 4
,1+4an 2
∴a2=
+1 4
=1 2
.3 4
(Ⅱ)∵
=bn,
+an1 4
∴an=bn2-
,代入an+1=an+1 4
+1 4
得到:1+4an 2
=(bn+b 2n+1
)2,1 2
∵bn>0,
∴bn+1-bn=
,所以数列{bn}是以b1=1 2
为首项,公差为1 2
的等差数列.bn=1 2
+(n-1)•1 2
=1 2
n.即数列{bn}的通项公式为bn=1 2
n.1 2
(Ⅲ)要使g(n)≥m(m∈R)对任意n>1,n∈N*都成立,只须m≤[g(n)min].
,∴g(n+1)-g(n)=2(∵g(n)=
+1 bn+1
+1 bn+2
+…+1 bn+3
=2(1 b2n
+1 n+1
+1 n+2
+…+1 n+3
)1 2n
+1 2n+1
-1 2n+2
)=1 n+1
>0,∴g(n)是增的,1 (2n+1)•(n+1)
∴[g(n)]min=g(2)=2•(
+1 3
)=1 4
,7 6
∴m的最大值为
.7 6