第八讲 多元函数积分学

  • 预备知识,三重积分,第一型积分(线,面),第二型积分(线Green公式,面Guass公式),stokes公式

一、预备知识

1."切一刀,投下来,转一圈"

  • 1 "切一刀"——曲面方程的切平面

    • :F(x,y,x)=0切平面P0为面上一点设\sum : F(x,y,x)=0 切平面 P_0为面上一点

    • 1.n={FxP0,FyP0,FzP0,}={A,B,C}1. \vec n = \{ F'_x|_{P_0}, F'_y|_{P_0}, F'_z|_{P_0}, \} = \{A,B,C\}

    • 2.写方程A(xx0)+B(yy0)+C(zz0)=02. 写方程 A(x-x_0)+B(y-y_0)+C(z-z_0) = 0

  • 2."投下来"——曲线(曲面)在坐标面上的投影曲线(平面区域)

    • L投下来LL 投下来\rightarrow L_投

    • Σ投下来D\Sigma 投下来\rightarrow D

    • 投影时,先消去z,在补上z=0投影时,先消去z,在补上z=0

  • 3."转一圈"——旋转曲面方程

    • $$1.提法:将\Gamma: \left{ \begin{array}{ll}

      F(x,y,z)=0\

      G(x,y,z)=0 \end{array} \right. 绕L: \ \frac {x-x_0}{l} = \frac {y-y_0}{m}+\frac {z-z_0}{n} 旋转一周所得曲面\Sigma$$

    • 2.求法2.求法

      • M0(x0,y0,z0)L,τ(l,m,n)L的法向量M_0(x_0,y_0,z_0)在L上, \vec \tau (l,m,n)是L的法向量

      • M1(x1.y1,z1)Γ,M1L旋转一周得纬圆,P(x,y,z)为纬圆上一点M_1(x_1.y_1,z_1)在\Gamma上,M_1绕L旋转一周得纬圆,P(x,y,z)为纬圆上一点

      • M0,M1,P构成圆锥M_0,M_1,P构成圆锥

      • $$\left{ \begin{array}{ll}

        (x_1-x_0)^2+(y_1-y_0)^2+(z_1-z_0)^2=(x-x_0)^2+(y-y_0)^2+(z-z_0)^2\l(x-x_1)+m(y-y_1)+n(z-z_1)=0\

        F(x_1,y_1,z_1)=0 \G(x_1,y_1,z_1)=0 \end{array} \right. \ \Rightarrow \Sigma: f(x,y,z)= 0 ——空间曲面方程 $$

2.场论初步

  • u=u(x,y)——数量场field没有向量方向,例如温度场u=u(x,y) —— 数量场 field 没有向量方向,例如温度场

  • u=u(x,y)——向量场,有方向,有大小如重力场\vec u = \vec u(x,y) —— 向量场,有方向,有大小 如重力场

  • 1.方向导数u=u(x,y)1.方向导数 u=u(x,y)

    • P0(x0,y0),P1(x0+tcosα,y0+tsinα),P0P1lP_0(x_0,y_0), P_1(x_0+t\cos\alpha,y_0+t\sin \alpha),P_0到P_1为\vec l

    • 1.定义法

      • ulP0limt0+u(x0+tcosα,y0+tsinα)u(x0,y0t\frac {\partial u}{\partial \vec l} | _{P_0} \triangleq \lim_{t\to 0^+} \frac{u(x_0+t\cos\alpha,y_0+t\sin \alpha) - u(x_0,y_0}{t}

    • 2.公式法

      • u(x,y)P0处可微,ulP0=uxP0cosα+uyP0sinα若u(x,y)在P_0处可微,\\则\frac {\partial u}{\partial \vec l} | _{P_0}=u'_x|_{P_0} \bullet \cos \alpha + u'_y|_{P_0} \bullet \sin \alpha

  • 2.梯度gradientu=u(x,y)2.梯度gradient u=u(x,y)

    • grad UP0={uxP0,uyP0}\overrightarrow {grad}\ U |_{P_0} =\{ u'_x|_{P_0} , u'_y|_{P_0} \}

  • 3.散度u=u(x,y,z)={P,Q,R}旋转3.散度 \vec u = \vec u(x,y,z) = \{P,Q,R\} 旋转

    • u=P(x,y,z)i+Qj+Rk\vec u = P(x,y,z)\vec i + Q\vec j + R\vec k

    • 散度div u=Px+Qy+Rz散度 div\ \vec u = \frac {\partial P}{\partial x} + \frac {\partial Q}{\partial y} + \frac {\partial R}{\partial z}

  • 4.旋度,三阶行列式4.旋度, 三阶行列式

    • $$ \vec rot \quad \vec u = \left| \begin{array}{ccc}

      \vec i & \vec j & \vec k \

      \frac{\partial }{\partial x} & \frac{\partial }{\partial y} & \frac{\partial }{\partial z} \

      P & Q & R

      \end{array} \right| $$

二、三重积分

1.定义

  • 类比二重积分

  • Df(x,y)dσ\iint_Df(x,y)d\sigma

  • Ωf(x,y,z)dv\iiint_{\Omega}f(x,y,z)dv

2.计算

  • 1.直角系与柱面系下

    • dv=dxdydz(直角系下)dv=dxdydz (直角系下)

    • dv=dθrdrdz(直角+=)dv=d\theta rdrdz (直角+极 = 柱)

    • 法一:先一后二法(先z后xy法,投影穿线法)

      • 后积先定限,限内画条线,先交写下限,后交写上限

      • 某个例子

      • I=Ωzdv=D1xy:x2+y21dσ12zdz+D2xy:1x2+y24dσx2+y22zdzI= \iiint_{\Omega}zdv = \iint_{D_{_1xy}:x^2+y^2\le 1}d\sigma \bullet \int_1^2 zdz \\+ \iint_{D_{_2xy}:1\le x^2+y^2\le 4}d\sigma \bullet \int_{\sqrt{x^2+y^2}}^2 zdz

    • 法二:(先xy后z的方法,定限截面法)

      • Ωzdv=12dzDxy:x2+y2z2zdσ\iiint_{\Omega}zdv = \int_1^2 dz \iint_{D_{xy}:x^2+y^2\le z^2}zd\sigma

  • 2.球面系下

    • dv=r2sinφdθdφdr记住即可dv = r^2\sin\varphi d\theta d\varphi dr \qquad 记住即可

    • $$\left{ \begin{array}{ll}

      x = r\sin\varphi \bullet \cos\theta \

      y = r\sin\varphi \bullet \sin\theta \ z = r\cos\varphi \end{array} \right.$$

    • I=Ωf(x,y,z)dv=θ1θ2dθφ1φ2dφr1r2f(x,y,z)r2sinφdr=θ1θ2dθφ1φ2dφr1r2f(rsinφcosθ,rsinφsinθ,rcosφ)r2sinφdrI = \iiint_{\Omega}f(x,y,z)dv \\ = \int_{\theta_1}^{\theta_2}d\theta \int_{\varphi_1}^{\varphi_2}d\varphi \int_{r_1}^{r_2}f(x,y,z)r^2\sin\varphi dr \\ = \int_{\theta_1}^{\theta_2}d\theta \int_{\varphi_1}^{\varphi_2}d\varphi \int_{r_1}^{r_2}f(r\sin\varphi \cos\theta, r\sin\varphi\sin\theta, r\cos\varphi)r^2\sin\varphi dr

三、第一型积分

1.第一型曲线积分(和定积分,二重积分,三重积分本质一致都是面积)

  • 1.定义

    • 对比定积分,底边由直线变曲线,dx变ds弧微分,L:y=y(x)

    • Lf(x,y)ds\int_Lf(x,y)ds

    • ds=(dx)2+(dy)2ds =\sqrt{ (dx)^2 + (dy)^2}

    • ds=1+(yx)2dxds =\sqrt{ 1 + (y'_x)^2}dx

  • 2.计算口诀:一投二代三计算

    • $$1.参数方程 L: \left{ \begin{array}{ll}

      x = x(t) \

      y = y(t) \end{array} \right. \qquad \alpha \le t \le \beta $$

      • Lf(x,y)ds=αβf(x(t),y(t))(x(t))2+(y(t))2dt\int_Lf(x,y)ds = \int_{\alpha}^{\beta}f(x(t),y(t))\sqrt {(x'(t))^2+(y'(t))^2}dt

    • $$2.显式方程 L: \left{ \begin{array}{ll}

      y = y(x) \

      (x = x) \end{array} \right. \qquad a \le x \le b $$

      • Lf(x,y)ds=abf(x,y(x))1+(y(x))2dx\int_Lf(x,y)ds = \int_a^bf(x,y(x))\sqrt {1+(y'(x))^2}dx

2.第一型曲面积分

  • 1.定义

    • 对比二重积分,底面变为曲面,dσ变为dS面密度,曲面质量对比二重积分,底面变为曲面,d\sigma 变为dS 面密度,曲面质量

    • Σf(x,y,z)dS\iint_{\Sigma}f(x,y,z)dS

    • dS=1+(zx)2+(zy)2dxdydS =\sqrt{1+(z'_x)^2+(z'_y)^2}dxdy

  • 2.计算口诀:一投二代三计算

    • Σ:z=z(x,y)f(x,y,z)dS=Dxyf(x,y,z(x,y))1+(zx)2+(zy)2dxdy\iint_{\Sigma:z=z(x,y)}f(x,y,z)dS \\ = \iint_{D_{xy}}f(x,y,z(x,y))\sqrt{1+(z'_x)^2+(z'_y)^2}dxdy

四、第二型积分

1.第二型曲线积分——无几何背景

1.概念

  • 给力场F=F(x,y)=P(x,y)i+Q(x,y)j给力场 \vec F=\vec F(x,y) = P(x,y)\vec i + Q(x,y)\vec j

  • 弧微分向量ds={dx,dy}=dxi+dyj弧微分向量d\vec s = \{dx,dy\}=dx\vec i+dy\vec j

  • dW=P(x,y)dx+Q(x,y)dydW=P(x,y)dx+Q(x,y)dy

  • W=LdW=LP(x,y)dx+Q(x,y)dy——物理意义做功W = \int_LdW = \int_LP(x,y)dx+Q(x,y)dy ——物理意义做功

2.计算

  • 1.直接法:一投二代三计算

    • $$1.参数方程 L: \left{ \begin{array}{ll}

      x = x(t) \

      y = y(t) \end{array} \right. \qquad t:\alpha \to \beta 起点到终点 $$

    • LP(x,y)dx+Q(x,y)dy=αβ(P(x(t),y(t))x(t)+Q(x(t),y(t))y(t))dt\int_LP(x,y)dx+Q(x,y)dy = \\ \int_{\alpha}^{\beta}(P(x(t),y(t))x'(t)+Q(x(t),y(t))y'(t))dt

  • 2.格林公式法(间接法,green公式)

    • 正方向,沿着正方向走,左手在D内

    • L+P(x,y)dx+Q(x,y)dy=D(QxPy)dxdy\oint_{L^+}P(x,y)dx+Q(x,y)dy \\=\iint_D(\frac{\partial Q}{\partial x}-\frac{\partial P}{\partial y})dxdy

    • $$成立要求: \left{ \begin{array}{ll}

      1.L封闭取正向 \

      1. P,Q,\frac{\partial Q}{\partial x},\frac{\partial P}{\partial y}在D中连续 \end{array} \right.$$

2.第二型曲面积分——无几何背景

1.概念

  • 给流场u=u(x,y,z)=P(x,y,z)i+Q(x,y,z)j+R(x,y,z)k给流场 \vec u = \vec u(x,y,z) = P(x,y,z)\vec i + Q(x,y,z)\vec j + R(x,y,z)\vec k

  • dS=dydzi+dzdxj+dxdykdΦ=Pdydz+Qdzdx+Rdxdyd\vec S = dydz\vec i + dzdx\vec j+ dxdy\vec k \\ \Rightarrow d\Phi = Pdydz+Qdzdx+Rdxdy

  • Φ=ΣPdydz+Qdzdx+Rdxdy——通量\Phi = \iint_{\Sigma}Pdydz+Qdzdx+Rdxdy ——通量

2.计算

  • 1.直接法:一投二代三计算

    • 一个一个算

    • Σ:z=z(x,y)R(x,y,z)dxdy=DxyR(x,y,z(x,y)(±dxdy)\iint_{\Sigma:z=z(x,y)}R(x,y,z)dxdy = \\ \iint_{D_{xy}}R(x,y,z(x,y)(\pm dxdy)

    • []指定n,k=锐角+dxdy[注] 指定\vec n_外,\vec k = 锐角 \Rightarrow +dxdy

    • []指定n,k=钝角dxdy[注] 指定\vec n_外,\vec k = 钝角 \Rightarrow -dxdy

  • 2.高斯公式法

    • ΣPdydz+Qdzdx+Rdxdy=Ω(Px+Qy+Rz)dv\oint\oint_{\Sigma_外}Pdydz+Qdzdx+Rdxdy = \\ \iiint_{\Omega}(\frac{\partial P}{\partial x} + \frac{\partial Q}{\partial y} + \frac{\partial R}{\partial z})dv

    • $$成立要求: \left{ \begin{array}{ll}

      1.\Sigma封闭取外侧 \

      1. P,Q,R,\frac{\partial P}{\partial x},\frac{\partial Q}{\partial y},\frac{\partial R}{\partial z}在\Omega中连续 \end{array} \right.$$

五、空间第二型曲线积分

1.斯托克斯公式

  • Ω为空间某区域,ΣΩ内分片光滑有向曲面片,L为逐段光滑的Σ的边界,其方向与Σ外法向符合右手定则设\Omega为空间某区域, \Sigma为\Omega内分片光滑有向曲面片, \\L为逐段光滑的\Sigma的边界,其方向与\Sigma外法向符合右手定则

  • $$\ointLPdx+Qdx+Rdz=\iint{\Sigma}\left| \begin{array}{ccc}

    \cos\alpha & \cos\beta & \cos\gamma \

    \frac{\partial }{\partial x} & \frac{\partial }{\partial y} & \frac{\partial }{\partial z} \

    P & Q & R

    \end{array} \right|dS\其中\vec n_0={\cos\alpha,\cos\beta,\cos\gamma}为\Sigma的单位法向量$$

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