问题 解答题
已知函数f(x)=
1
2
ax2-(a+1)x+ln(x+1)

(Ⅰ)如果f(x)在区间(1,2)不单调,求a的取值范围;
(Ⅱ)如果a>0,设函数g(x)=f(x)+ax,求函数g(x)的极大值.
答案

(If′(x)=

ax2-x- a
x+1

设h(x)=ax2-x-a=0的两个根为x1,x2

由韦达定理得x1•x2=1

∵f(x)在区间(1,2)不单调

∴h(x)=0在区间(1,2)上h(x)=0有且仅有一个根,另一个根小于1,

则h(1)h(2)<0

即(a-1-a)(4a-2-a)<0

解得a>

2
3

(II)g′(x)=

ax[x-(
1
a
-1)]
x+1

①当a=1时,函数g(x)无极值

②当a>1时,在(-1,

1
a
-1)上,g′(x)>0,g(x)单调递增,

(

1
a
-1,0)上,g′(x)<0,g(x)单调递减

在(0,+∞)时,g′(x)>0,g(x)单调递增

∴当x=

1
a
-1时,g(x)取得极大值为
1
2
a-
1
2a
-1-lna

③当0<a<1时,函数g(x)在区间(-1,0)和(

1
a
-1,+∞)上是增函数,在区间(0,
1
a
-1)
是减函数

所以函数g(x)的极大值为g(0)=0

单项选择题
完形填空
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小题1:
A.injureB.ruinC.hurtD.harm
小题2:
A.throughB.fromC.inD.for
小题3:
A.latterB.rightC.oppositeD.free
小题4:
A.knockedB.overtookC.missedD.left
小题5:
A.laughingB.throwingC.glancingD.yelling
小题6:
A.wonderedB.smiledC.ignoredD.guessed
小题7:
A.friendlyB.angryC.frightenedD.disappointed
小题8:
A.showedB.suggestedC.explainedD.hated
小题9:
A.expectationB.passengersC.garbageD.goods
小题10:
A.turnsB.pushesC.holdsD.piles
小题11:
A.roadsB.childrenC.youD.dustbins
小题12:
A.healthyB.happyC.pitifulD.wealthy
小题13:
A.spreadB.shareC.explainD.contribute
小题14:
A.surpriseB.pleasureC.doubtD.regrets
小题15:
A.funnyB.importantC.strangeD.embarrassing
小题16:
A.rememberB.forgetC.valueD.appreciate
小题17:
A.enjoyB.exchangeC.recall D.imagine
小题18:
A.inspireB.takeC.mendD.notice
小题19:
A.lettingB.consistingC.makingD.dreaming
小题20:
A.distinguishB.deserveC.deliverD.determine