第五章 定积分
一.理解定积分的概念,掌握定积分的性质(积分中值定理),几何意义;
1.定积分的定义: 设函数
在
上有界,在
中任意插入若干个分点
把区间
分成
个小区间
各个小区间的长度依次为
在每个小区间
上任取一点
,作函数值
与小区间长度
乘积
并作出和
,记
,如果不论对
怎样分法,也不论在小区间
上点
怎样取法,只要当
时,和S总趋于确定的极限I,这时我们称这个极限I为函数
在
上的定积分,记为
.
注(1)定积分是一个数,仅与
及[a,b]有关.
(2)
在[a,b]上可积的充分条件是
在[a,b]上连续(或者是
在[a,b]有界且只有有限个间断点).
2.定积分的性质
(1)
(2)
(不论
的相对位置如何)
(3)
(4)
,则
(
)
推论1:
, 则
(
推论2:
(
)
(5) 设
和
是
在
上的最大值和最小值, 则
.
(6) (积分中值定理) 若
在
上连续,则存在
使
.
注:
在
上的平均值 即“平均高度”为
.
3.定积分的几何意义
(1)
,
示曲边梯形的面积;
(2)
,
表示曲边梯形面积的负值;
(3)
在
上可取正值和负值,则在
轴上方的面积赋
,在
轴下方的面积 赋
,
的几何意义介于
轴,
,
及
的图 形之间的各 部分面积的代数和.
(4)由
轴,
,
及
围成图形的面积为
.
例1.设
,
在
上的图形如右图,其中三块面积
,
,
,求
在
上的最大值和最小值.
解:
,
,所以 函数
驻点为
,
又
,
,
,
所以 最大值为3,最小值为
.
二. 理解变上限定积分定义的函数,会求它的导数.
1.定义:称
为
在
上的变上限积分.
2.性质:若函数
在
上连续,则
在
上具有导数,且
(
).
注 (1)
;
(2)
.
3.定理:若函数
在
上连续,则
是
在
上的一个原函数.
例2. (1)
(2)
(3)
EMBED Equation.3
EMBED Equation.3
EMBED Equation.3
(4)
EMBED Equation.3
EMBED Equation.3 .
例3.(1)设
,求
在
的表达式.
解:当
时
当
时
EMBED Equation.3
.
.
(2)设函数
,求
。
解 当
时,
当
时,
由于
在
处连续得
,
.
(3)已知函数
,
,且在
上有
,求
.
解
因为
在
是连续的,故
,
.
即
,从而
.
例4.(1)
EMBED Equation.3
EMBED Equation.3
EMBED Equation.3 .
(2)设函数
连续且
,求极限
.
解. 原式=
EMBED Equation.3
EMBED Equation.3
EMBED Equation.3 .
(3)求由
所确定的隐函数
对
的导数.
解:
EMBED Equation.3 .
练习:设
,其中
在
某邻域内可导,且
,
,求
.
解:
EMBED Equation.3
EMBED Equation.3
EMBED Equation.3
EMBED Equation.3
EMBED Equation.3
EMBED Equation.3
EMBED Equation.3 .
例5.设
为连续函数,证明
.
证明:
EMBED Equation.3
EMBED Equation.3
EMBED Equation.3 ,
令
得
得证.
例6.求方程
在区间
内实根的个数?
解:令
在
单调增加且
在区间
内仅有一个根.
练习:设
,则
( )
(A)正常数; (B)负常数; (C)恒为零; (D)不为常数
解
EMBED Equation.3
EMBED Equation.3
EMBED Equation.3
或
EMBED Equation.3
EMBED Equation.3 .
例7.设函数
在
内连续,且
,
试证(1)若
为偶函数,则
也是偶函数; (2)若
单调减少,则
单调增加.
证(1)
EMBED Equation.3
,
是偶函数.
(2)
EMBED Equation.3
EMBED Equation.3
在
与
之间
当
时
EMBED Equation.3
当
时
EMBED Equation.3
单调增加.
例8. 设
是连续函数,
是
的原函数,则
(A)当
是奇函数时,
必是偶函数;
(B)当
是偶函数时,
必是奇函数;
(C)当
是周期函数时,
是周期函数;
(D)当
是单调函数时,
必是单调增函数; 答案(A)
例9(1)已知
,计算
.
解:
EMBED Equation.3
EMBED Equation.3 .
(2)设
是区间
上的单调、可导函数,其中
为
的反函数,且满足
,求
.
解:
EMBED Equation.3
EMBED Equation.3
EMBED Equation.3
EMBED Equation.3 ,
EMBED Equation.3
EMBED Equation.3
在等式
中,令
得
, 所以
.
所以
.
EMBED Equation.3 .
三.掌握
公式和定积分换元法与分部积分法。
1.
公式:设
在
上连续,则
EMBED Equation.3
EMBED Equation.3 ;
2.定积分的换元法:
EMBED Equation.3
EMBED Equation.3 ;
3.定积分的分部积分法:
;
几个常用公式
(1)奇、偶函数及周期函数的积分性质.
a)若
在
上连续,且为偶函数,则
b)若
在
上连续,且为奇函数,则
.
c)设
是以
为周期的连续函数,则
的值与
无关.
(2)
(3)
.
例10.(1)
.
(2)
EMBED Equation.3
(3)
,其中
EMBED Equation.3
EMBED Equation.3
EMBED Equation.3 .
(4)
EMBED Equation.3
(5)
EMBED Equation.3
EMBED Equation.3 .
(6)
(7)
EMBED Equation.3
EMBED Equation.3 。
例11.(1)
EMBED Equation.3
EMBED Equation.3
EMBED Equation.3
EMBED Equation.3 .
(2)
EMBED Equation.3
EMBED Equation.3
=
.
例12.设
,求
.
解:
EMBED Equation.3
,
.
例13.设
,(1)证明:
是以
为周期的周期函数;
(2)求
的值域.
解(1)
=
是以
为周期的函数
(2)求
的值域可以转化为
在
的最大值和最小值。
EMBED Equation.3
在
上最大值
最小值
,
值域
.
练习.设
是可导的正函数且
,令
,
EMBED Equation.3 ,(1)证明
单调增加;(2)求
的最小值点;(3)将
的最小值看作
的函数,若它等于
,求
解 (1)
EMBED Equation.3
EMBED Equation.3
EMBED Equation.3
.
(2)
EMBED Equation.3
EMBED Equation.3
EMBED Equation.3
EMBED Equation.3 .
EMBED Equation.3
EMBED Equation.3 .
EMBED Equation.3
EMBED Equation.3 .
.
(3)
EMBED Equation.3
EMBED Equation.3
,
.
四.了解广义积分的概念,会计算广义积分。
无穷区间上的广义积分:
,
,
无界函数的广义积分:
,
主要结果:
:当
时,收敛;当
时,发散.
:当
时,收敛;当
时,发散.
例14 (1)
EMBED Equation.3
EMBED Equation.3
(2)
EMBED Equation.3 =
(3)
EMBED Equation.3
EMBED Equation.3
EMBED Equation.3 .
(4)
EMBED Equation.3
EMBED Equation.3
EMBED Equation.3
EMBED Equation.3 .
练习:
(既是无穷区间又是无界函数的广义积分)
解. 令
EMBED Equation.3
五.定积分的应用:用定积分表达和计算一些几何量与物理量(平面图形的面积、旋转体的体积及侧面积、平行截面面积为已知的立体体积、平面曲线的弧长、功、引力、压力)及函数的平均值.
微元法:
(1)根据问题的具体情况,选取一个变量,例如
为积分变量,并确定所求量的变化区间
(2)任取
,所求量A在
上的部分量
的近似值为
——微元.
(3) 所求量
1.平面图形的面积:((1)先画出草图;(2)选择积分变量;确定积分限;(3)取面积微元;
(4)计算定积分得面积).
由曲线
与直线
所围成的平面图形的面积为
例15.计算抛物线
与直线
所围成的图形的面积.
解1(1)选
为积分变量
(2)
(3)
解2. 选
为积分变量
,
.
2.已知平行截面的面积,求立体体积:
.
特别地:由连续曲线
,直线
及X轴所围成的平面图形绕X轴旋转一周而成的立体的体积
.
例16.(1)求由平面图形
分别绕y轴及直线
旋转一周所得立体的体积.
解:绕
轴
EMBED Equation.3 .
绕
EMBED Equation.3 .
(2)求椭圆
所围成的图形的面积.
解:
.
练习:计算由摆线
的一拱,直线
所围成的图形分别绕
轴及
轴旋转而成的旋转体的体积
解:
EMBED Equation.3
EMBED Equation.3
EMBED Equation.3 .
练习.已知曲线L的方程为
.(1)讨论L的凹凸性;(2)过点
引L的切线,求切点
,并写出切线的方程;(3)求此切线与L(对应于
的部分)及X轴所围成的平面图形的面积.
解 (1)
,
,
(2)
EMBED Equation.3
EMBED Equation.3 .
(3)
EMBED Equation.3 .
例17.设直线
与抛物线
所围成的图形的面积为
,它们与直线
所围成的图形面积为
,并且
(1)试确定a的值,使
达到最小,并求出最小值。
(2)求该最小值所对应的平面图形绕x轴旋转一周所决定立体体积.
解(1)(a)当
时
EMBED Equation.3
EMBED Equation.3
EMBED Equation.3
又
即
为最小值
(b)当
时,
EMBED Equation.3
,
在
单减小.
故
时
为最小值。 综上所述最小值
.
(2) 当
时图形绕
轴旋转一周的立体的体积
EMBED Equation.3 .
3.平面曲线的弧长(数三不要求)
(1)L:
(2)L:
4.旋转体的侧面积(数三不要求)
例18.设有曲线
,过原点作切线,求由此曲线、切线、x轴围成的平面图形绕x轴旋转一周所得旋转体的表面积.
解:设切点坐标为
EMBED Equation.3
切点坐标
EMBED Equation.3
,
.
6.物理应用:(看课本)(数三不要求)
(1)变力做功:
例19.设质点
在平行于X轴的力
作用,使质点从1沿X轴移到3,求力F所做功.
练习.半径为R的球沉入水中,球上部与水面相切,球的比重为
现将球从水中取出,需作多少功?
解(球缺公式
) 球从
到
运动过程中受力
是
的函数
球重-浮力
EMBED Equation.3 .
注:水下球缺高度
.
(2)水压力
练习.设有一底为8cm,高为6cm的等腰三角形片,铅直地沉没在水中,顶在上,底在下且与水面平行,而顶离水面3cm,试求它每面所受的压力?
(3)引力:
例20.设有密度均匀,长度为
,质量为
的质线AB,求质线AB对质量为
的质点的引力,假设(1)质点在BA的延长线上距A为a处;(2)质点在AB的垂直平分线上距AB为a处.
解(1)(a)所求引力落在
(b)任取
(c)
EMBED Equation.3 .
(2)(a)
(b)任取
,
(c)
EMBED Equation.3
.
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