(1)设x =0是f (x )的极值点,求m ,并讨论f (x )的单调性; (2)当m ≤2时,求证:f (x )>0. 解:(1)f ′(x )=e x -1
x +m
,由f ′(0)=0,得m =1, 所以f ′(x )=e x -
1x +1,f ″(x )=e x +1(x +1)2
>0, 又由f ′(0)=0,所以当x ∈(-1,0)时,f ′(x )<0, 当x ∈(0,+∞)时,f ′(x )>0.
所以函数f (x )在(-1,0)上单调递减,在(0,+∞)上单调递增. (2)证明:依题意,f ′(x )=e x -1
x +m
. 令f ′(x 0)=0,则e x 0=1x 0+m >0,且f ″(x )=e x +1
(x +m )2
>0,所以函数f ′(x )在区间(-m ,+∞)上单调递增.
则当x ∈(-m ,x 0)时,f ′(x 0)<0;
当x ∈(x 0,+∞)时,f ′(x )>0,故函数f (x )在(-m ,x 0)上单调递减,在(x 0,+∞)上单调递增.
所以f (x )min =f (x 0)=e x 0-ln(x 0+m ).
又x 0满足方程e x 0=
1
x 0+m
, 则f (x 0)=e x 0-ln(x 0+m )=
1x 0+m -ln e -x 0=x 0+1x 0+m =x 0+m +1
x 0+m
-m ≥①2
(x 0+m )·1
x 0+m -m =2-m ≥②0( 不等号①取等号的条件是⎩
⎪⎨⎪⎧
x 0=0,m =1,不等号②取等号的条件是m =2 ).
由于不等号①、②不能同时取等号,故f (x 0)>0,即f (x )min >0,因此f (x )>0. 3.已知函数f (x )=ax +b
x +c (a >0)的图象在点(1,f (1))处的切线方程为y =x -1. (1)试用a 表示出b ,c ;
(2)若f (x )≥ln x 在[1,+∞)恒成立,求a 的取值范围. 解:(1)b =a -1,c =1-2a .
(2)题设即“a ≥ln x +1x -1
x +1x -2
(x >1),或a ≥x ln x -x +1
(x -1)2(x >1) 恒成立”.
用导数可证函数g (x )=1
2(x -1)2+(x -1)-x ln x (x ≥1)是增函数(只需证g ′(x )=x -ln x
-1≥0(x ≥1)恒成立,再用导数可证),
所以g (x )≥g (1)=0(x ≥1), 当且仅当x =1时g (x )=0,得x ln x -x +1(x -1)2<12(x >1),li m x →1+ x ln x -x +1
(x -1)2
=12.
所以若a ≥
x ln x -x +1(x -1)2
(x >1)恒成立,则a ≥1
2,
即a 的取值范围是⎣⎡⎭
⎫1
2,+∞. 4.(2019·安徽二校联考)已知函数f (x )=ln x -a
x -m (a ,m ∈R)在x =e(e 为自然对数的底数)时取得极值,且有两个零点记为x 1,x 2.
(1)求实数a 的值,以及实数m 的取值范围; (2)证明:ln x 1+ln x 2>2.
解:(1)f ′(x )=1
x ·x -(ln x -a )x 2=a +1-ln x
x 2,
由f ′(x )=0,得x =e a +
1,且当0时,f ′(x )>0,
当x >e a
+1
时,f ′(x )<0,
所以f (x )在x =e a
+1
时取得极值,
所以e a +
1=e ,解得a =0.
所以f (x )=
ln x
x -m (x >0),f ′(x )=1-ln x x 2
, 函数f (x )在(0,e)上单调递增,在(e ,+∞)上单调递减,f (e)=1
e
-m .
又x →0(x >0)时,f (x )→-∞;x →+∞时,f (x )→-m ,f (x )有两个零点x 1,x 2, 故⎩⎪⎨⎪⎧
1e -m >0,-m <0,解得0.
所以实数m 的取值范围为⎝⎛⎭
⎫0,1
e . (2)证明:不妨设x 1ln x 1=mx 1,
ln x 2=mx 2
.
则ln x 1x 2=m (x 1+x 2),ln x 2
x 1=m (x 2-x 1)⇒m =ln
x 2
x 1x 2-x 1
.欲证ln x 1+ln x 2>2,只需证ln
x 1x 2>2,
只需证m (x 1+x 2)>2,即证
x 1+x 2x 2-x 1ln x 2
x 1
>2. 即证1+
x 2x 1x 2x 1-1ln x 2x 1>2,设t =x 2
x 1>1,
则只需证ln t >2(t -1)
t +1.
即证ln t -2(t -1)
t +1>0.
记u (t )=ln t -
2(t -1)
t +1
(t >1), 则u ′(t )=1t -4
(t +1)2=(t -1)2t (t +1)2>0.
所以u (t )在(1,+∞)上单调递增, 所以u (t )>u (1)=0,所以原不等式成立, 故ln x 1+ln x 2>2.
5.(2016·全国卷Ⅰ)已知函数f (x )=(x -2)e x +a (x -1)2有两个零点. (1)求a 的取值范围;
(2)设x 1,x 2是f (x )的两个零点,证明:x 1+x 2<2. 解:(1)f ′(x )=(x -1)e x +2a (x -1)=(x -1)(e x +2a ). ①设a =0,则f (x )=(x -2)e x ,f (x )只有一个零点. ②设a >0,则当x ∈(-∞,1)时,f ′(x )<0;