问题 解答题

若存在常数k和b,使得函数f(x)和g(x)在它们的公共定义域上的任意实数x分别满足:f(x)≥kx+b和g(x)≤kx+b,则称直线l:y=kx+b为函数f(x)和g(x)的“隔离直线”.已知f(x)=x2,g(x)=2elnx.

(I)求F(x)=f(x)-g(x)的极值;

(II)函数f(x)和g(x)是否存在隔离直线?若存在,求出此隔离直线的方程,若不存在,请说明理由.

答案

(1)∵F(x)=f(x)-g(x)=x2-2clnx(x>0),

∴F′(x)=2x-

2c
x
=(2x2-2c)/x=
2(x-
e
)(x+
e
x

令F′(X)=0,得x=

e

当0<x<

e
时,F′(X)<0,X>
e
时,F′(x)>0

故当x=

e
时,F(x)取到最小值,最小值是0

(2)由(1)可知,函数f(x)和g(x)的图象在x=

e
处有公共点,因此存在f(x)和g(x)的隔离直线,那么该直线过这个公共点,设隔离直线的斜率为k.则隔离直线方程为y-e=k(x-
e
,即y=kx-k
e
+e

由f(x)≥kx-k

e
+e(x⊂R),可得x2-kx-k
e
+e,

由f(x)≥kx-k

e
+e(x⊂R),可得x2-kx+k
e
-e≥0当x⊂R恒成立,

则△=k2-4k

e
+4e=(k-2
c
2≤0,只有k=2
e
,此时直线方程为:y=2
e
x-e,

下面证明g(x)≤2

e
x-eexx>0时恒成立

令G(x)=2

e
x-e-g(x)=2
e
x-e-2elnx,

G′(X)=2

c
-
2c
x
=(2
c
x-2c)/x=2
c
(x-
e
)/x,

当x=

e
时,G′(X)=0,当0<x<
e
时G′(X)>0,

则当x=

e
时,G(x)取到最小值,极小值是0,也是最小值.

所以G(x)=2

e
x-e-g(x)≥0,则g(x)≤2
e
x-e当x>0时恒成立.

∴函数f(x)和g(x)存在唯一的隔离直线y=2

e
x-e

单项选择题
单项选择题


Nine states and the District of Columbia are doing away with the sales tax on items such as clothes, shoes and even notebooks over the next few weeks, just in time for back-to- school shopping.
Most of the promotions last only a few days, so shoppers will have to act quickly to get a tax break. The tax holidays, which have already expired in two other states, apply to small and large items. For example, the tax break applies to any school supply that costs $15 or less in New Mexico. In Massachusetts, it covers most retail purchases of $2,500 or less. Stores may offer additional savings because these events "give retailers an opportunity to have a sale on top of what the state is doing," says Verenda Smith, government affairs associate at the Federation of Tax Administrators.
No industry-wide figures are available about how much consumers save annually from these tax breaks. But Texas estimates that shoppers will save $47.4 million in taxes this year, nearly a 3 percent increase from the previous year. Massachusetts says shoppers saved roughly $10 million in taxes during its 2005 event.
States that cast aside these promotions when a slow economy pinched state budgets are now reviving them in hopes of stimulating local economies. Some states also believe the gain in consumer goodwill helps balance out the loss in tax revenue.
In some cases, tax losses are minimal because serious shoppers don’t stop at clothes and books. "While states give up sales tax, they usually break even on sales-tax collection," says J. Craig Shearman, a vice-president at the National Retail Federation.
Sophie Beckmann, a certified public accountant at A. G. Edwards in St. Louis, says she’ll avoid that temptation by making a list of necessities. On the Missouri resident’s shopping list: notebooks, pencils, glue and three or four outfits for her son, who is entering the fifth grade this month. She plans to pocket any tax savings. "When you start buying more and spending more just because of the savings, then you’re not doing yourself a favor," Beckmann says.

According to the passage, some states choose to provide tax breaks because they want to ______ .

A.cast aside these promotions

B.pinch state budget books

C.slow down local economy

D.encourage their economies