浅析常数变易法PPT课件
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常数变易法
常数变易法是求解复杂问题中经常采用的一种方法,它既可以帮助我们求解复杂问题,又可以帮助我们节省时间,提高效率。
但是,要想有效地使用常数变易法,我们需要对它有全面的认识和理解,并能够熟练掌握运用它的相关技巧。
首先,我们来了解它的定义,常数变易法就是从现有的函数中求解函数变形的方法,它的关键就是利用函数的变易性,将原始的函数变形为一个简单的函数,让求解问题更加容易。
例如,如果我们要求解一个立方函数,我们可以利用常数变易法,将其变形为一个平方函数,这样就可以用更简单的方式来求解。
其次,在掌握常数变易法的时候,我们需要学习它的基本原理,主要是利用二次函数的“常数变易”原理,即一次函数可以表示为一次函数与常数相乘的形式。
换句话说,利用“常数变易”原理,我们可以将复杂的函数变形为更为简单的函数,从而求解复杂的函数。
此外,为了有效地运用常数变易法,我们还需要掌握一些算法,才能够更加高效地求解复杂函数。
比如,我们可以用分治算法来求解复杂的函数,而且分治算法可以从另一个角度来分析函数,从而使函数的求解更加容易。
总的来说,常数变易法是一种解决复杂问题的高效方法,它可以帮助我们通过变易函数的方式节省时间,提高效率。
但是,如果要有效地使用常数变易法,我们还需要学习它的基本原理、熟练掌握它的算法,这样才能够有效地求解复杂的函数。
线性微分方程的常数变易法线性微分方程是微积分中重要的研究对象,常数变易法是解线性微分方程的一种常用方法。
本文将介绍线性微分方程以及常数变易法的基本概念和步骤。
1. 线性微分方程的定义和形式线性微分方程是指形如y'' + p(x)y' + q(x)y = r(x)的微分方程,其中p(x)、q(x)和r(x)为已知函数,y为未知函数。
一阶线性微分方程可以表示为y' + p(x)y = q(x)。
2. 常数变易法的基本思想常数变易法是对齐次线性微分方程的解进行求解的一种方法。
首先求得齐次线性微分方程的通解,然后利用常数变易法找出非齐次线性微分方程的一个特解,将通解和特解相加得到非齐次线性微分方程的通解。
3. 常数变易法的步骤步骤一:求齐次线性微分方程的通解对于齐次线性微分方程y'' + p(x)y' + q(x)y = 0,我们可以先求得其特征方程。
特征方程是通过将y替换为е^(rx)得到的方程,其中r为常数。
解特征方程可以得到一组线性无关的解,它们的线性组合就是齐次线性微分方程的通解。
步骤二:求非齐次线性微分方程的特解对于非齐次线性微分方程y'' + p(x)y' + q(x)y = r(x),我们假设其特解为y = u(x)v(x),其中u(x)为常数,v(x)为齐次线性微分方程的通解。
将特解y代入非齐次线性微分方程,可以得到一个关于u(x)的方程,若能解出u(x)的具体形式,则可以得到非齐次线性微分方程的一个特解。
步骤三:求非齐次线性微分方程的通解将齐次线性微分方程的通解和非齐次线性微分方程的特解相加,即可得到非齐次线性微分方程的通解。
4. 常数变易法的应用举例以一阶线性微分方程y' + p(x)y = q(x)为例,根据常数变易法的步骤,首先求得齐次线性微分方程y' + p(x)y = 0的通解,然后假设特解为y =u(x)v(x),将特解代入非齐次线性微分方程,解出u(x)的具体形式,最后将通解和特解相加即可得到非齐次线性微分方程的通解。
常数变易法的原理
常数变易法是一种数学方法,用于求解特定类型的问题。
它的原理是通过假设一个未知数为常数,并在后续计算中逐步调整这个常数,以便解决问题。
使用常数变易法的关键是找到一个适当的常数,使得问题的解可以用这个常数来表示。
一般来说,常数经过调整后可以使问题简化,或者使得解的形式更加容易处理。
在使用常数变易法时,首先需要假设一个常数,并将其视为未知数,然后将这个常数代入问题的表达式或方程中进行计算。
根据计算结果,通过适当调整常数的值,逐步逼近或找到问题的解。
常数变易法的思想通常用于求解微积分、微分方程和变分问题等数学领域中的一些特殊问题。
它的目的是通过假设一个常数来简化问题,使得求解变得更加容易和直观。
总之,常数变易法是一种使用常数作为未知数,并通过逐步调整常数的值来求解问题的方法。
它可以简化问题,使得计算更加方便,从而得到问题的解。
Learn General Secretary on "two to learn a" strengthening "four Consciousnesses" important speech caused a strong reaction in the country. Time, watching "red treasure", the origin of building the party back to power, how to strengthen services for the masses, improve party cohesion, fighting to become the grass-roots party members and masses hot topic. Grass-roots party organizations "two" is to strengthen the service of party members and cadres, the pioneer spirit. Distribution of grass-roots party organizations in all walks of people, clothing, shelter, which belongs to the nerve endings of the party organization and comments reputation has a direct perception of the masses. Strengthen the party ahead of the "pedal" spirit; strengthen the party members and cadres "success does not have to be me" and "the first to bear hardships, the last to" service spirit to set the party's positive image among the people is important. Grass-roots party organizations "two" is to cleanse all people not happy not to see "stereotypes", establish the honest faithful, diligent faith for the people. No need to avoid mentioning that, some members of our party can not stand the "money," corrosion of temptation, thin, Xu Zhou, such abuse and corrupt bribery, malfeasance borers, and rats. Two, is to clean up, thin, Xu, Zhou's solution to restore the party's fresh and natural, solid and honest work style. Cleansing "take, eat, card," undesirable and behaviour, "cross, hard and cold, push" attitude. Grass-roots party organizations "two" is to strengthen the sense of ordinary party members, participating in consciousness, unity consciousness. For reasons known, members of grass-roots party branches less mobile, less resources, and the construction of party organizations have some lag. Two studies, is to focus on the grass-roots party branches "loose, soft, loose" problem, advance the party members and cadres, "a gang working", "Hong Kong report." Strong cleanup actions, style and rambling, presumptuous "unqualified" party members, pays special attention to party members and cadres "joining party of thought" problem. "Party building" is obtained in the long-term development of our party's historical experience accumulated. Two is our party under the new historical conditions, strengthen the party's construction of a new "rectification movement." Grass-roots party organizations should always catch the hard work, results-oriented. Two educational outcomes are long-term oriented and become an important impetus for the work. "Two" should have three kinds of consciousness "two" study and education, basic learning lies in the doing. Only the Constitution address the series of party rules, and do solid work, be qualified party members had a solid ideological basis. Only the "learning" and "do" real unity, to form a "learn-learn-do-do" the virtuous cycle, and ultimately achieve the fundamental objective of education. This requires that the Organizationlearning education, need three kinds of consciousness: one is to establish an integrated awareness. "Learning" and "do" what car isTwo-wheel, bird wings, need to go hand in hand, one end can be neglected. Communist theoretician and man. Only by closely combining theory and practice together in order to truly realize their value. "Learning" is the Foundation, the Foundation is not strong, shaking; " "Is the key to net to net thousands of accounts. "Two" education, "" lay the basis, going to "do" the key grip, so that the "learning" and "doing" back to standard, so that the majority of party members "learn" learning theory of nutrients, in the "doing" practice party's purposes. Second, to establish a sense of depth. "Learning" and "do" not Chu drawn, entirely different, but the organic unity of the whole. "Two" learning education, we need to explore integrating "learning" in "do", exhibit "do" in "Science". To avoid the "learning" into simple room instruction, "do" into a monotone for doing. Should exploration "learn" in the has "do", "do" in the has "learn" of education and practice of carrier, makes general grass-roots members can in "learn" in the has "do" of achievements sense, in "do" in the has "learn" of get sense, real makes party of theory brain into heart, put for people service concept outside of Yu shaped. Third, to adhere to long-term the awareness. Style construction on the road forever, "two" had to catch the long-term. "Two" study and education, by no means, assault-style wind-sport, but the recurrent education within the party. In recent years, the party's mass line education practice and "three-three" special education in grass-roots borne rich fruits, vast numbers of party members and cadres withstood the baptism of the spirit. "Two" greater need to focus on longer hold long-term, to establish and perfect the effective mechanism of the education, focusing on the creation of long-term education, strive to make the vast number of party members to maintain their vanguard Color, maintain the party's advanced nature and purity. Awareness-raising, antennas and atmosphere – a discussion on how leading cadres of party members "two" current, "two" activity is in full swing up and down the country, party cadres as a "key minority" is both a barometer and impetus. The "two" meaning enough deep, is to determine the party cadres can resolve to study hard first. In the "two" in the process, some cadres of himself, standing long, high awareness, that Constitution Party rules is simple, its not worth bothering some party cadres think speak series has nothing to do with the grass-roots work, water business learning series of speeches seen as window dressing. These "lazy, casual, and decadent" ideas learning lacks motivation, a serious impediment to "two" effect. John Stuart Mill once said, only a basic element of human thought patterns change dramatically, human destiny can make great improvement. The same, only party members and2.2 一阶线性方程与常数变易公式(First order linear differential equationand constant variation formula )[教学内容] 1. 认识一阶线性齐次方程和一阶线性非齐次方程; 2.介绍一阶线性非齐次方程的常数变易公式; 3. 介绍电学知识和基尔霍夫定律; 4. 认识Bernoulli 方程及其通过变量替换化为一阶线性方程的解法; 5. 介绍其他可化为一阶线性方程的例子.[教学重难点] 重点是知道一阶线性非齐次方程的解法,难点是如何根据方程的形式引入新的变量变换使得新方程为一阶线性方程.[教学方法] 自学1、4;讲授2、3 课堂练习 [考核目标]1. 熟练运用常数变易公式;2. 知道计算和一些三角函数恒等式; 3. 知道电⎰dx bx sin e ax 学一些知识,如电容电流公式、电感电压公式和基尔霍夫定律; 4. 知道溶液混合问题建模; 5. 认识Bernoulli 方程并会经过适当变换化为线性方程求解. 6. 知道交换自变量和因变量化非线性方程为一阶线性方程.1. 认识一阶线性齐次方程和一阶线性非齐次方程(First order (non)homogeneous linear differential equation )(1) 称形如的方程为一阶线性齐次方程,其中连续;y p(x)dxdy=p(x)称形如的方程为一阶线性非齐次齐次方程,其中连续且q(x)y p(x)dxdy+=q(x) p(x),不恒为零.q(x)(2) 当时,改写为0y ≠y p(x)dxdy=,其中表示P(x)的一个原1C dx p(x)|y |ln ,dx p(x)y dy dx, p(x)y dy +===⎰⎰⎰⎰dx p(x)函数(antiderivative). 因此,通解(general solution)为,y p(x)dxdy =1C p(x)dx e C ~ ,eC ~y =⎰±=此外y=0也是解. 综上,的解为为任意常数.y p(x)dxdy =C ,eC y p(x)dx⎰=(3) 常数变易法:如何求的解呢?q(x)y p(x)dxdy+=假定上述线性非齐次方程有如下形式的解 ,则代入原方程来确定C(x),⎰=p(x)dxeC(x)y ,q(x)p(x)C(x)e e p(x) C(x)e (x)' C dxdy p(x)dxp(x)dx p(x)dx +⎰=⎰+⎰=即,,此处C 为q(x)e(x)' C p(x)dx=⎰C q(x)dx eC(x) q(x), e(x)' C p(x)dx-p(x)dx+⎰=⎰=⎰-表示在时刻t 时电容两端电压,则根据电学知识,电容两端电量电阻两端电压为R I=. 由基尔霍夫定律知,dtdUR . 改写为 ,这是一个一阶线性非齐次方程0dt dU =RC EU RC 1dt dU +⋅-=ELearn General Secretary on "two to learn a" strengthening "four Consciousnesses" important speech caused a strong reaction in the country. Time, watching "red treasure", the origin of building the party back to power, how to strengthen services for the masses, improve party cohesion, fighting to become the grass-roots party members and masses hot topic. Grass-roots party organizations "two" is to strengthen the service of party members and cadres, the pioneer spirit. Distribution of grass-roots party organizations in all walks of people, clothing, shelter, which belongs to the nerve endings of the party organization and comments reputation has a direct perception of the masses. Strengthen the party ahead of the "pedal" spirit; strengthen the party members and cadres "success does not have to be me" and "the first to bear hardships, the last to" service spirit to set the party's positive image among the people is important. Grass-roots party organizations "two" is to cleanse all people not happy not to see "stereotypes", establish the honest faithful, diligent faith for the people. No need to avoid mentioning that, some members of our party can not stand the "money," corrosion of temptation, thin, Xu Zhou, such abuse and corrupt bribery, malfeasance borers, and rats. Two, is to clean up, thin, Xu, Zhou's solution to restore the party's fresh and natural, solid and honest work style. Cleansing "take, eat, card," undesirable and behaviour, "cross, hard and cold, push" attitude. Grass-roots party organizations "two" is to strengthen the sense of ordinary party members, participating in consciousness, unity consciousness. For reasons known, members of grass-roots party branches less mobile, less resources, and the construction of party organizations have some lag. Two studies, is to focus on the grass-roots party branches "loose, soft, loose" problem, advance the party members and cadres, "a gang working", "Hong Kong report." Strong cleanup actions, style and rambling, presumptuous "unqualified" party members, pays special attention to party members and cadres "joining party of thought" problem. "Party building" is obtained in the long-term development of our party's historical experience accumulated. Two is our party under the new historical conditions, strengthen the party's construction of a new "rectification movement." Grass-roots party organizations should always catch the hard work, results-oriented. Two educational outcomes are long-term oriented and become an important impetus for the work. "Two" should have three kinds of consciousness "two" study and education, basic learning lies in the doing. Only the Constitution address the series of party rules, and do solid work, be qualified party members had a solid ideological basis. Only the "learning" and "do" real unity, to form a "learn-learn-do-do" the virtuous cycle, and ultimately achieve the fundamental objective of education. This requires that the Organizationlearning education, need three kinds of consciousness: one is to establish an integrated awareness. "Learning" and "do" what car isTwo-wheel, bird wings, need to go hand in hand, one end can be neglected. Communist theoretician and man. Only by closely combining theory and practice together in order to truly realize their value. "Learning" is the Foundation, the Foundation is not strong, shaking; " "Is the key to net to net thousands of accounts. "Two" education, "" lay the basis, going to "do" the key grip, so that the "learning" and "doing" back to standard, so that the majority of party members "learn" learning theory of nutrients, in the "doing" practice party's purposes. Second, to establish a sense of depth. "Learning" and "do" not Chu drawn, entirely different, but the organic unity of the whole. "Two" learning education, we need to explore integrating "learning" in "do", exhibit "do" in "Science". To avoid the "learning" into simple room instruction, "do" into a monotone for doing. Should exploration "learn" in the has "do", "do" in the has "learn" of education and practice of carrier, makes general grass-roots members can in "learn" in the has "do" of achievements sense, in "do" in the has "learn" of get sense, real makes party of theory brain into heart, put for people service concept outside of Yu shaped. Third, to adhere to long-term the awareness. Style construction on the road forever, "two" had to catch the long-term. "Two" study and education, by no means, assault-style wind-sport, but the recurrent education within the party. In recent years, the party's mass line education practice and "three-three" special education in grass-roots borne rich fruits, vast numbers of party members and cadres withstood the baptism of the spirit. "Two" greater need to focus on longer hold long-term, to establish and perfect the effective mechanism of the education, focusing on the creation of long-term education, strive to make the vast number of party members to maintain their vanguard Color, maintain the party's advanced nature and purity. Awareness-raising, antennas and atmosphere – a discussion on how leading cadres of party members "two" current, "two" activity is in full swing up and down the country, party cadres as a "key minority" is both a barometer and impetus. The "two" meaning enough deep, is to determine the party cadres can resolve to study hard first. In the "two" in the process, some cadres of himself, standing long, high awareness, that Constitution Party rules is simple, its not worth bothering some party cadres think speak series has nothing to do with the grass-roots work, water business learning series of speeches seen as window dressing. These "lazy, casual, and decadent" ideas learning lacks motivation, a serious impediment to "two" effect. John Stuart Mill once said, only a basic element of human thought patterns change dramatically, human destiny can make great improvement. The same, only party members and解:设I(t)表示时刻t 时电感线圈上电流强度,则由电学知识有,电感线圈两端电压为. 由基尔霍夫定律知,闭合回路电压降为零. 于是 . 改写为dt dIL 0dt dIL I R E =--, 这是一个一阶线性非齐次方程. sin wt U L1L I R dt dI m +-=记, 由常数变易公式得到,wt sin L Uq(t) ,L R p(t)m =-=)C ~dt sin wt LU e (e )C ~q(t)dt e(eI(t)m L RtL Rt p(t)dtp(t)dt⎰⎰+=+⎰⎰=--.ba btcos b bt sin a e bt))isin bt (cos e b a ib)(a Im()e ib a 1Im()dt e Im(dt )Im(e e dt bt sin e 22ata22ib)t (a ib)t (a ibt at at +-=+⋅+-=+===++⎰⎰⎰22t LR mLRt m m LRtw (R/L) wt)cos w sin wt LR(e LU dt sin wt e LUdt sin wt L U e+-==⎰⎰令,2222w(R/L)wφsin ,w(R/L)R/L φ cos +-=+=于是由知,B sin A cos B cos A sin B)sin(A +=+,于是.22t LR m m LRtw (R/L)φ)sin(wt e L U dt sin wt L U e++=⎰L Rt 22m e C ~w (R/L)φ)sin(wt LU I(t)-+++=再注意到初始条件I(0)=0,,因此,22m0022m w(R/L)φsin L U C ~0,C ~e e w (R/L)φsin LU I(0)+-==++=.t LR22m22m ew(R/L)sin(φLUw (R/L)φ)sin(wt L U I(t)-+-++=)练习23. (1) 求; (2) 改写为,给出dt bt cos e at ⎰t cos b sin t a +θ)sin(t ba 122++所满足的条件. (3) 由 Euler 公式和推导出:θb sin i b cos e ib +=R b a, ,e e e b)i(a b i a i ∈=⋅+和b asin sin b cos a cos b)cos(a b,sin a cos b cos a sin b)sin(a -=++=+learning education, need three kinds of consciousness: one is to establish an integrated awareness. "Learning" and "do" what car isTwo-wheel, bird wings, need to go hand in hand, one end can be neglected. Communist theoretician and man. Only by closely combining theory and practice together in order to truly realize their value. "Learning" is the Foundation, the Foundation is not strong, shaking; " "Is the key to net to net thousands of accounts. "Two" education, "" lay the basis, going to "do" the key grip, so that the "learning" and "doing" back to standard, so that the majority of party members "learn" learning theory of nutrients, in the "doing" practice party's purposes. Second, to establish a sense of depth. "Learning" and "do" not Chu drawn, entirely different, but the organic unity of the whole. "Two" learning education, we need to explore integrating "learning" in "do", exhibit "do" in "Science". To avoid the "learning" into simple room instruction, "do" into a monotone fordoing. Should exploration "learn" in the has "do", "do" in the has "learn" of education and practice of carrier, makes general grass-roots members can in "learn" in the has "do" of achievements sense, in "do" in the has "learn" of get sense, real makes party of theory brain into heart, put for people service concept outside of Yu shaped. Third, to adhere to long-term the awareness. Style construction on the road forever, "two" had to catch the long-term. "Two" study and education, by no means, assault-style wind-sport, but the recurrent education within the party. In recent years, the party's mass line education practice and "three-three" special education in grass-roots borne rich fruits, vast numbers of party members and cadres withstood the baptism of the spirit. "Two" greater need to focus on longer hold long-term, to establish and perfect the effective mechanism of the education, focusing on the creation of long-term education, strive to make the vast number of party members to maintain their vanguard Color, maintain the party's advanced nature and purity. Awareness-raising, antennas and atmosphere – a discussion on how leading cadres of party members "two" current, "two" activity is in full swing up and down the country, party cadres as a "key minority" is both a barometer and impetus. The "two" meaning enough deep, is to determine the party cadres can resolve to study hard first. In the "two" in the process, some cadres of himself, standing long, high awareness, that Constitution Party rules is simple, its not worth bothering some party cadres think speak series has nothing to do with the grass-roots work, water business learning series of speeches seen as window dressing. These "lazy, casual, and decadent" ideas learning lacks motivation, a serious impediment to "two" effect. John Stuart Mill once said, only a basic element of human thought patterns change dramatically, human destiny can make great improvement. The same, only party members and3. Bernoulli 方程及其解法称形如为Bernoulli 方程. R n ,y q(x)y p(x)dxdyn ∈+=解法:当时,改写原方程,0y ≠1n , n)q(x)(1y p(x) n)(1dxdy y n)-(1n -1n -≠-+-=令,这是一个一阶线性非齐次方程.n)q(x)(1n)p(x)u (1dx du ,y u n1-+-==-例31 求解方程.2y x xy6dx dy -=解:经过观察,原方程是一个Bernoulli 方程, n=2. (1)当时,改写原方程为,令,则0y ≠ x 2)(1y x62)(1dx dy 2)y-(1212---=--21y u -=. 由常数变易公式得到, x u x6dx du +-=.6276-dx x6dx x6xC8x C)dx x (x )C xdx e(eu(x)+=+=+⎰⎰=⎰⎰-返回原变量得到.62xC8x y 1+=(2) 当y=0时,容易验证也是原方程的解. 0y =作业26. 求解方程(1); (2). 33y x y x dxdy=+1y(1) ,y xy 'y x 22==-4. 交换自变量和因变量化非线性方程为一阶线性方程例32. 求解(1); (2). 2y 2x y dx dy -=33yx xy 1dx dy -=解:(1) 这是一个一阶方程,非线性方程,不是Bernoulli 方程.(a) 当时,交换自变量和因变量而改写原方程为 . 这是一个0y ≠y x y2y y 2x dy dx 2-=-=一阶线性方程. 由常数变易公式得到, ,C)y)dy (e(ex dy y2dy y2+-⎰⎰=⎰-即 为所求方程的通积分.|)y |ln (C y C)y)dy (y1(y x 222-=+-=⎰(b) 当y=0时,已验证y=0也是原方程的一个解. (2) 结合Bernoulli 方程来完成,留作练习.作业27. 求解方程(1); (2) . 3y x ydx dy +=y 2y x dx dy 22+=learning education, need three kinds of consciousness: one is to establish an integrated awareness. "Learning" and "do" what car isTwo-wheel, bird wings, need to go hand in hand, one end can be neglected. Communist theoretician and man. Only by closely combining theory and practice together in order to truly realize their value. "Learning" is the Foundation, the Foundation is not strong, shaking; " "Is the key to net to net thousands of accounts. "Two" education, "" lay the basis, going to "do" the key grip, so that the "learning" and "doing" back to standard, so that the majority of party members "learn" learning theory of nutrients, in the "doing" practice party's purposes. Second, to establish a sense of depth. "Learning" and "do" not Chu drawn, entirely different, but the organic unity of the whole. "Two" learning education, we need to explore integrating "learning" in "do", exhibit "do" in "Science". To avoid the "learning" into simple room instruction, "do" into a monotone for doing. Should exploration "learn" in the has "do", "do" in the has "learn" of education and practice of carrier, makes general grass-roots members can in "learn" in the has "do" of achievements sense, in "do" in the has "learn" of get sense, real makes party of theory brain into heart, put for people service concept outside of Yu shaped. Third, to adhere to long-term the awareness. Style construction on the road forever, "two" had to catch the long-term. "Two" study and education, by no means, assault-style wind-sport, but the recurrent education within the party. In recent years, the party's mass line education practice and "three-three" special education in grass-roots borne rich fruits, vast numbers of party members and cadres withstood the baptism of the spirit. "Two" greater need to focus on longer hold long-term, to establish and perfect the effective mechanism of the education, focusing on the creation of long-term education, strive to make the vast number of party members to maintain their vanguard Color, maintain the party's advanced nature and purity. Awareness-raising, antennas and atmosphere – a discussion on how leading cadres of party members "two" current, "two" activity is in full swing up and down the country, party cadres as a "key minority" is both a barometer and impetus. The "two" meaning enough deep, is to determine the party cadres can resolve to study hard first. In the "two" in the process, some cadres of himself, standing long, high awareness, that Constitution Party rules is simple, its not worth bothering some party cadres think speak series has nothing to do with the grass-roots work, water business learning series of speeches seen as window dressing. These "lazy, casual, and decadent" ideas learning lacks motivation, a serious impediment to "two" effect. John Stuart Mill once said, only a basic element of human thought patterns change dramatically, human destiny can make great improvement. The same, only party members and5. 一些一阶线性方程的理论(1)考虑方程,其中p(x), q(x)都是以w>0为周期的连续函数. 用常数q(x)y p(x)dxdy=+变易公式证明:(a) 若,则方程任一非零解都以w 为周期的周期函数充要条件是0q(x)≡p(x)的平均值 (b) 若不恒为零,则方程有唯一w 周期解充要.0p(x)dx w 1(x)p w==⎰q(x)条件是, 试求出此解. (参见丁同仁、李承治《常微分方程教程》0p(x)dx w 1(x)p w≠=⎰P36 习题5, 6)。
2.2 一阶线性方程与常数变易公式(First order linear differential equationand constant variation formula )[教学内容] 1. 认识一阶线性齐次方程和一阶线性非齐次方程; 2.介绍一阶线性非齐次方程的常数变易公式; 3. 介绍电学知识和基尔霍夫定律; 4. 认识Bernoulli 方程及其通过变量替换化为一阶线性方程的解法; 5. 介绍其他可化为一阶线性方程的例子.[教学重难点] 重点是知道一阶线性非齐次方程的解法,难点是如何根据方程的形式引入新的变量变换使得新方程为一阶线性方程.[教学方法] 自学1、4;讲授2、3 课堂练习 [考核目标]1. 熟练运用常数变易公式;2. 知道⎰dx bx sin e ax 计算和一些三角函数恒等式; 3. 知道电学一些知识,如电容电流公式、电感电压公式和基尔霍夫定律; 4. 知道溶液混合问题建模; 5. 认识Bernoulli 方程并会经过适当变换化为线性方程求解. 6. 知道交换自变量和因变量化非线性方程为一阶线性方程.1. 认识一阶线性齐次方程和一阶线性非齐次方程(First order (non)homogeneous linear differential equation ) (1) 称形如y p(x)dxdy=的方程为一阶线性齐次方程,其中p(x)连续; 称形如q(x)y p(x)dxdy+=的方程为一阶线性非齐次齐次方程,其中q(x) p(x),连续且q(x)不恒为零. (2) 当0y ≠时,改写y p(x)dxdy=为 1C dx p(x )|y |ln ,dx p(x )y dy dx , p(x )y dy +===⎰⎰⎰,其中⎰dx p(x)表示P(x)的一个原函数(antiderivative). 因此,y p(x)dxdy=通解(general solution)为1C p(x)dx e C ~ ,eC ~y =⎰±=,此外y=0也是解. 综上,y p(x)dxdy =的解为C ,eC y p(x)dx⎰=为任意常数. (3) 常数变易法:如何求q(x)y p(x)dxdy+=的解呢? 假定上述线性非齐次方程有如下形式的解 ⎰=p(x)dxeC(x )y ,则代入原方程来确定C(x),q(x )p(x )C(x )e e p(x ) C(x )e (x )' C dxdy p(x)dxp(x)dx p(x)dx +⎰=⎰+⎰=, 即q(x )e(x )' C p(x)dx=⎰,C q(x )dx eC(x ) q(x ), e(x )' C p(x)dx-p(x)dx+⎰=⎰=⎰-,此处C 为任意常数,⎰⎰q(x )dx ep(x)dx-为函数q(x )ep(x)dx-⎰一个原函数.综上,一阶线性非齐次方程的通解为⎰⎰⎰⎰+⎰=+⎰⋅⎰=q(x )dx eeCeC)q(x )dx e(ey(x )p(x)dx-p(x)dxp(x)dxp(x)dx-p(x)dx.2. 一些实际应用例子(Applications ) 例28. 电容器的充电和放电模型RC 电路:假定开始电容C 上没有电荷,电容两端电压为0,合上开关1后,电池E 对电容C 开始充电,电池电压为E ,电阻阻值为R ,电容C 两端电压逐渐上升. 写出充电过程中,电容C 两端电压随时间变化的规律.解:设U(t)表示在时刻t 时电容两端电压,则根据电学知识,电容两端电量Q=U C ,电流I =dt dU C dt dQ =, 电阻两端电压为R I=dtdUR . 由基尔霍夫定律知,闭合回路上压降为零. 即有0dt dU RC U E =--. 改写为 RC EU RC 1dt dU +⋅-=,这是一个一阶线性非齐次方程. 记RCEq(t) ,RC 1p(t)=-=, 由常数变易公式得到, C~e E )C ~(Ee e )C ~dt RCE e (e )C ~q(t)dt e(eU(t)RC tRC t RC t RC t RC t p(t)dtp(t)dt----+=+=+=+⎰⎰=⎰⎰再注意到初始条件U(0)=0,-E C ~0,C ~e Ee U(0)0==+=,因此,RC tEe E U(t)--=.例29. 考察如下RL 电路图,设电源E 的电压为0 U sin wt,U E m m >=为常数,求电感线圈上电流I 随时间的变化规律,设t=0时,I=0.解:设I(t)表示时刻t 时电感线圈上电流强度,则由电学知识有,电感线圈两端电压为dtdI L . 由基尔霍夫定律知,闭合回路电压降为零. 于是 0dtdIL I R E =--. 改写为sin wt U L1L I R dt dIm +-=, 这是一个一阶线性非齐次方程. 记wt sin L Uq(t) ,L R p(t)m =-=, 由常数变易公式得到,)C ~dt sin wt LU e (e )C ~q(t)dt e(eI(t)m L RtLRt p(t)dtp(t)dt⎰⎰+=+⎰⎰=--.b a bt cos b bt sin a e bt))isin bt (cos e b a ib)(a Im()e ib a 1Im()dt e Im(dt )Im(e e dt bt sin e 22at a 22ib)t(a ib)t (a ibt at at +-=+⋅+-=+===++⎰⎰⎰22t LR m LRtm m LRt w(R/L) wt)cos w sin wt L R(e LU dt sin wt e LUdt sin wt L U e+-==⎰⎰令2222w(R/L)w φsin ,w(R/L)R/L φ cos +-=+=,于是由B sin A cos B cos A sin B)sin(A +=+知,22t LR mm LRt w (R/L)φ)sin(wt e L U dt sin wt L U e++=⎰,于是LRt 22m e C ~w(R/L)φ)sin(wt L U I(t)-+++=.再注意到初始条件I(0)=0,22m0022m w(R/L)φsin L U C ~0,C ~e e w (R/L)φsin LU I(0)+-==++=,因此,t LR22m22m ew(R/L)sin(φLUw (R/L)φ)sin(wt L U I(t)-+-++=).练习23. (1) 求dt bt cos e at ⎰; (2) 改写 t cos b sin t a +为θ)sin(t ba 122++,给出θ所满足的条件. (3) 由 Euler 公式b sin i b cos e ib+=和R b a, ,e e e b)i(a bi ai ∈=⋅+推导出:b asin sin b cos a cos b)cos(a b,sin a cos b cos a sin b)sin(a -=++=+和b))sin(a b)(sin(a 21b cos a sin -++=, b))cos(a b)(cos(a 21b cos a cos -++=.作业24. (1) 如例28中RC 电路图,设E=10V , R=100Ω, C=0.01 F, 开始时刻电容C 上电压为零并在此刻合上开关1,问经过多长时间电容C 两端电压为V 5U 1=?(2)如下RL 电路图,设E, R, L 均为正的常数,求开关闭合后电路中电流强度I(t),假定I(0)=0.例30. 溶液混合问题:设容积为V (单位3m )的密封容器装着某种溶液如下图,从A 以速度r (单位/s m 3)流入浓度为0C e >(常数)的相同溶液,经充分混合后在B 以相同速度r 流出容器, 假设时刻t=0时,容器溶液浓度为0,问容器中浓度随时间变化的规律.解:设时刻t 时容器溶液浓度为C(t),且C(0)=0,则由溶质出入平衡,也即流入等于流出,由微元法建立如下等式:V C(t))Δt)(C(t C(t)Δt r C Δt r e -+≈-,即e C VrC V r dt dC +-=. (以下略)作业25. 假设伊利湖的存水量为34m 1048⨯,从休伦湖流入和从安大略湖流出的速度都是每年34m 1035⨯,在t=0时刻,伊利湖的污染物浓度时休伦湖的5倍. 如果流出的水是完全混合好的湖水,问使得伊利湖的污染物浓度减少到休伦湖2倍需要多少时间?(假定休伦湖污染物浓度为常数0C e >) 3. Bernoulli 方程及其解法称形如R n ,y q(x)y p(x)dxdyn ∈+=为Bernoulli 方程. 解法:当0y ≠时,改写原方程1n , n)q(x)(1y p(x) n)(1dxdy y n)-(1n -1n -≠-+-=, 令n)q(x)(1n)p(x)u (1dx du ,y u n 1-+-==-,这是一个一阶线性非齐次方程.例31 求解方程2y x xy6dx dy -=.解:经过观察,原方程是一个Bernoulli 方程, n=2. (1)当0y ≠时,改写原方程为 x 2)(1y x62)(1dx dy 2)y-(1212---=--,令21y u -=,则 x u x6dx du +-=. 由常数变易公式得到, 6276-dx x6dx x6xC8x C)dx x (x )C x dx e(eu(x )+=+=+⎰⎰=⎰⎰-.返回原变量得到62xC8x y 1+=.(2) 当y=0时,容易验证0y =也是原方程的解. 作业26. 求解方程(1)33y x y x dxdy=+; (2)1y(1) ,y x y 'y x 22==-. 4. 交换自变量和因变量化非线性方程为一阶线性方程 例32. 求解(1)2y 2x y dx dy -=; (2)33y x x y 1dx dy -=. 解:(1) 这是一个一阶方程,非线性方程,不是Bernoulli 方程.(a) 当0y ≠时,交换自变量和因变量而改写原方程为 y x y2y y 2x dy dx 2-=-=. 这是一个一阶线性方程. 由常数变易公式得到, C)y)dy (e(ex dy y2dy y2+-⎰⎰=⎰-,即 |)y |ln (C y C)y)dy (y1(y x 222-=+-=⎰为所求方程的通积分. (b) 当y=0时,已验证y=0也是原方程的一个解.(2) 结合Bernoulli 方程来完成,留作练习.作业27. 求解方程(1)3yx ydx dy +=; (2) y 2y x dx dy 22+=. 5. 一些一阶线性方程的理论 (1)考虑方程q(x)y p(x)dxdy=+,其中p(x), q(x)都是以w>0为周期的连续函数. 用常数变易公式证明:(a) 若0q(x)≡,则方程任一非零解都以w 为周期的周期函数充要条件是p(x)的平均值.0p(x)dx w1(x)p w0==⎰ (b) 若q(x)不恒为零,则方程有唯一w 周期解充要条件是0p(x)dx w1(x)p w0≠=⎰, 试求出此解. (参见丁同仁、李承治《常微分方程教程》P36 习题5, 6)。