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幼苗
共回答了18个问题采纳率:77.8% 举报
(1)因为g(x)=f(x)-ax 2 -x=ax 2 +ln(x+1)-ax 2 -x=ln(x+1)-x(x>-1),
所以 g ′ (x)=
1
x+1 -1=-
x
x+1 (x>-1) ,
当-1<x<0时,g ′ (x)>0,当x>0时,g ′ (x)<0,
故函数g(x)的单调递增区间为(-1,0),单调递减区间为(0,+∞).
g(x) max =g(0)=ln1=0.
(2)因为当x∈[0,+∞)时,不等式f(x)≤x恒成立,即ax 2 +ln(x+1)-x≤0恒成立,
设g(x)=ax 2 +ln(x+1)-x(x≥0),只需g(x) max ≤0即可.
g ′ (x)=2ax+
1
x+1 -1=
x[2ax+(2a-1)]
x+1 ,
①当a=0时, g ′ (x)=
-x
x+1 ,当x>0时,g ′ (x)<0,函数g(x)在(0,+∞)上单调递减,故g(x)≤g(0)=0成立.
②当a>0时,由 g ′ (x)=
x[2ax+(2a-1)]
x+1 =0 ,因x∈[0,+∞),所以 x=
1
2a -1 ,
1°若
1
2a -1<0 ,即a>
1
2 时,在区间(0,+∞)上,g ′ (x)>0,则g(x)在(0,+∞)上单调递增,
g(x)在[0,+∞)上无最大值,此时不满足条件;
2°若
1
2a -1≥0 ,即0<a ≤
1
2 时,函数g(x)在 (0,
1
2a-1 ) 上单调递减,在区间 (
1
2a-1 ,+∞) 上单调递增,同样g(x)在[0,+∞)上无最大值,不满足条件.
③当a<0时,由 g ′ (x)=
x[2ax+(2a-1)]
x+1 ,∵x∈[0,+∞),∴2ax+(2a-1)<0,
∴g ′ (x)<0,故函数g(x)在[0,+∞)上单调递减,故g(x)≤g(0)=0成立.
综上所述,实数a的取值范围是(-∞,0].
(3)由(1)知ln(x+1)≤x,令x=
1
n 2 ,所以 ln(1+
1
n 2 )≤
1
n 2 =
1
n•n <
1
(n-1)n =
1
n-1 -
1
n ,
所以 ln(1+
1
2 2 )+ln(1+
1
3 2 )+…+ln(1+
1
n 2 ) < (1-
1
2 )+(
1
2 -
1
3 )+…+(
1
n-1 -
1
n )=1-
1
n <1 ,
所以 ln(1+
1
2 2 )(1+
1
3 2 )…(1+
1
n 2 )<1=lne ,
所以 (1+
1
2 2 )(1+
1
3 2 )…(1+
1
n 2 )<e .
1年前
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