第二章第10课时知能演练轻松闯关

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1.设y =-2e x sin x ,则y ′等于( )
A .-2e x cos x
B .-2e x sin x
C .2e x sin x
D .-2e x (sin x +cos x )
解析:选D.∵y =-2e x sin x ,
∴y ′=(-2e x )′sin x +(-2e x )·(sin x )′
=-2e x sin x -2e x cos x =-2e x (sin x +cos x ).
2.曲线f (x )=x ln x 在点x =1处的切线方程为( )
A .y =2x -2
B .y =2x +2
C .y =x -1
D .y =x +1
解析:选C.f ′(x )=ln x +1,f ′(1)=1,f (1)=0.切线方程为y =1×(x -1),即y =x -1,故选
C. 3.(2012·绵阳质检)设函数f (x )=13ax 3+bx (a ≠0),若f (3)=3f ′(x 0),则x 0=________. 解析:由已知f ′(x )=ax 2+b ,又f (3)=3f ′(x 0),则有9a +3b =3ax 20+3b ,所以x 20=3,则x 0=± 3.
答案:± 3
4.已知函数f (x )的图象在点M (1,f (1))处的切线方程是2x
-3y +1=0,则f (1)+f ′(1)=________. 解析:依题意得2×1-3f (1)+1=0,即f (1)=1,f ′(1)=23,则f (1)+f ′(1)=53
. 答案:53
一、选择题
1.下列函数求导运算正确的个数为( )
①(3x )′=3x log 3e ;②(log 2x )′=1x ·ln2;③(e x )′=e x ; ④⎝⎛⎭
⎫1ln x ′=x . A .1 B .2
C .3
D .4
解析:选B.求导运算正确的有②③,故选B.
2.函数y =x 2cos x 的导数为( )
A .y ′=2x cos x -x 2sin x
B .y ′=2x cos x +x 2sin x
C .y ′=x 2cos x -2x sin x
D .y ′=x cos x -x 2sin x
解析:选A.y ′=(x 2)′cos x +x 2(cos x )′=2x cos x -x 2sin x .故选A.
3.函数f (x )=ln x x 在点(x 0,f (x 0))处的切线平行于x 轴,则f (x 0)=( ) A .-1e B.1e
C.1e 2 D .e 2 解析:选B.与x 轴平行的切线,其斜率为0,所以f ′(x 0)=1x 0·x 0-ln x 0x 20=1-ln x 0x 20=0,故x 0
=e ,∴f (x 0)=1e
. 4.已知f 1(x )=sin x +cos x ,f n +1(x )是f n (x )的导函数,即f 2(x )=f 1′(x ),f 3(x )=f 2′(x ),…,f n +
1(x )=f n ′(x ),n ∈N *,则f 2012(x )=( )
A .-sin x -cos x
B .sin x -cos x
C .-sin x +cos x
D .sin x +cos x
解析:选B.∵f 1(x )=sin x +cos x ,∴f 2(x )=f 1′(x )=cos x -sin x ,∴f 3(x )=f 2′(x )=-sin x -cos x ,∴f 4(x )=f 3′(x )=-cos x +sin x ,∴f 5(x )=f 4′(x )=sin x +cos x ,∴f n (x )是以4为周期的函数,∴f 2012(x )=f 4(x )=sin x -cos x ,故选B.
5.曲线y =x 3在点(1,1)处的切线与x 轴及直线y =1所围成的三角形的面积为( )
A.112
B.16
C.13
D.12
解析:选B.求导得y ′=3x 2,所以y ′=3x 2|x =1=3,所以曲线y =x 3在点(1,1)处的切线方程
为y -1=3(x -1),结合图象易知所围成的三角形是直角三角形,三个交点的坐标分别是⎝⎛⎭⎫23,0,(1,0),(1,1),于是三角形的面积为12×⎝⎛⎭⎫1-23×1=16
,故选B. 二、填空题
6.函数y =sin x x
的导数为________. 解析:y ′=(sin x )′x -sin x ·x ′x 2=x cos x -sin x x 2. 答案:x cos x -sin x x 2 7.(2012·开封调研)若函数f (x )=12
x 2-ax +ln x 存在垂直于y 轴的切线,则实数a 的取值范围是________.
解析:∵f (x )=12x 2-ax +ln x ,∴f ′(x )=x -a +1x
. ∵f (x )存在垂直于y 轴的切线,∴f ′(x )存在零点,
x +1x -a =0,∴a =x +1x
≥2. 答案:[2,+∞)
8.已知函数f (x )的导函数为f ′(x ),且满足f (x )=3x 2+2x ·f ′(2),则f ′(5)=________.
解析:对f (x )=3x 2+2xf ′(2)求导,
得f ′(x )=6x +2f ′(2).
令x =2,得f ′(2)=-12.
再令x =5,得f ′(5)=6×5+2f ′(2)=6.
答案:6
三、解答题
9.求下列函数的导数:
(1)y =(1-x )(1+1x );(2)y =tan x ; (3)y =(1+sin x )2.
解:(1)∵y =(1-x )(1+1x )=1x -x =x 12--x 12, ∴y ′=(x 12-)′-(x 12)′=-12x 32--12
x 1
2-. (2)y ′=(sin x cos x )′=(sin x )′cos x -sin x (cos x )′cos 2x
=cos x cos x -sin x (-sin x )cos 2x
=1cos 2x
. (3)y ′=[(1+sin x )2]′=2(1+sin x )·(1+sin x )′
=2(1+sin x )·cos x =2cos x +sin2x .
10.已知函数f (x )=12
x 2-a ln x (a ∈R).若函数f (x )的图象在x =2处的切线方程为y =x +b ,求a ,b 的值.
解:因为f ′(x )=x -a x
(x >0), 又f (x )在x =2处的切线方程为y =x +b ,
所以⎩⎪⎨⎪⎧ 2-a ln2=2+b ,2-a 2=1,
解得a =2,b =-2ln2. 11.设f (x )是定义在R 上的奇函数,且当x ≥0时,f (x )=2x 2.
(1)求x <0时,f (x )的表达式;
(2)令g (x )=ln x ,问是否存在x 0,使得f (x )、g (x )在x =x 0处的切线互相平行?若存在,请求出x 0的值;若不存在,请说明理由.
解:(1)当x <0时,-x >0,
f (x )=-f (-x )=-2(-x )2=-2x 2.
(2)若f (x )、g (x )在x =x 0处的切线互相平行,
则f ′(x 0)=g ′(x 0),则f ′(x 0)=4x 0=g ′(x 0)=1x 0
, 解得x 0=±12,又由题知x 0>0,∴得x 0=12。