diff --git a/194.tex b/194.tex new file mode 100644 index 0000000..4a5f946 --- /dev/null +++ b/194.tex @@ -0,0 +1,139 @@ +\magnification=1200 + +\noindent +{\it George Weigt --- Advanced Calculus Homework \#10} + +\beginsection Page 312, problem 1a. + +Evaluate the line integral +$$\int_C z\,dx+x\,dy+y\,dz$$ +where $C$ is the curve that for $0\le t\le 2\pi$ is parameterized as +$$\eqalign{ +x&=\cos t\cr +y&=\sin t\cr +z&=t\cr +}$$ + +\bigskip +\noindent +Solution: We have +$$\eqalign{ +dx&=-\sin t\,dt\cr +dy&=\cos t\,dt\cr +dz&=dt\cr +}$$ +Hence +$$\eqalign{ +\int_C z\,dx+x\,dy+y\,dz +&=\int_0^{2\pi}\left(-t\sin t+\cos^2 t+\sin t\right)dt\cr +&=\left(t\cos t-\sin t+{t\over2}+{\sin t\cos t\over2}-\cos t\right)\bigg|_0^{2\pi}\cr +&=(2\pi-0+\pi+0-1)-(0-0+0+0-1)\cr +&=3\pi\cr +}$$ + +\beginsection Page 312, problem 1b. + +Evaluate the line integral +$$\int_C x^2\,dx-xz\,dy+y^2\,dz$$ +where $C$ is a straight line form $(1,0,1)$ to $(2,3,2)$. + +\bigskip +\noindent +Solution: Let $0\le t\le 1$. +Then $C$ can be parameterized as follows. +$$\eqalign{ +x&=1+t\cr +y&=3t\cr +z&=1+t\cr +}$$ +We have +$$dx=dt,\qquad dy=3\,dt,\qquad dz=dt$$ +Hence +$$\eqalign{ +\int_C x^2\,dx-xz\,dy+y^2\,dz +&=\int_0^1[(t+1)^2-3(t+1)^2+9t^2]\,dt\cr +&=\int_0^1(7t^2-4t-2)\,dt\cr +&=\left({7t^3\over3}-2t^2-2t\right)\bigg|_0^1\cr +&=-{5\over3}\cr +}$$ + +\beginsection Page 312, problem 1d. + +Evaluate the line integral +$$\int_C u_T\,ds$$ +where ${\bf u}=2xy^2z{\bf i}+2x^2yz{\bf j}+x^2y^2{\bf k}$, +and $C$ is the circle $x=\cos t$, $y=\sin t$, $z=2$, directed by increasing $t$. + +\bigskip +\noindent +Solution: We have +$$dx=-\sin t\,dt,\qquad dy=\cos t\,dt,\qquad dz=0$$ +hence +$$\eqalign{ +I&=\int_C 2xy^2z\,dx+2x^2yz\,dy+x^2y^2\,dz\cr +&=\int_0^{2\pi} 4(\cos t)(\sin^2 t)(-\sin t)\,dt+4(\cos^2 t)(\sin t)(\cos t)\,dt\cr +&=4\int_0^{2\pi}(-\cos t\sin^3 t+\cos^3 t\sin t)\,dt\cr +&=4\int_0^{2\pi}\sin t\cos t\,(-\sin^2 t+\cos^2 t)\,dt\cr +}$$ +Now for some trigonometric gymnastics. We have +$$\sin t\cos t={\sin2t\over 2}\qquad\cos^2t-\sin^2t=\cos 2t$$ +hence +$$4\sin t\cos t\,(-\sin^2 t+\cos^2 t)=2\sin2t\cos2t +=\sin4t$$ +Therefore +$$I=\int_0^{2\pi}\sin4t\,dt=-{\cos 4t\over4}\bigg|_0^{2\pi}=0$$ + +\beginsection Page 312, problem 1e. + +Evaluate the line integral +$$\int_C u_T\,ds$$ +where ${\bf u}=\mathop{\rm curl}{\bf v}$, ${\bf v}=y^2{\bf i}+z^2{\bf j}+x^2{\bf k}$, +and $C$ is the path $x=2t+1$, $y=t^2$, $z=1+t^3$, $0\le t\le 1$, directed by increasing $t$. + +\bigskip +\noindent +Solution: We have +$$\mathop{\rm curl}{\bf v} +=\left(\matrix{ +\partial x^2/\partial y-\partial z^2/\partial z\cr +\partial y^2/\partial z-\partial x^2/\partial x\cr +\partial z^2/\partial x-\partial y^2/\partial y\cr +}\right)=\left(\matrix{-2z\cr-2x\cr-2y}\right) +$$ +Therefore +$$\int_C u_T\,ds=-2\int_C z\,dx+x\,dy+y\,dz$$ +From the given parameteric functions we have +$$dx=2\,dt,\qquad dy=2t\,dt,\qquad dz=3t^2\,dt$$ +Hence +$$\eqalign{ +\int_C u_T\,ds +&=-2\int_0^1 (1+t^3)(2\,dt)+(2t+1)(2t\,dt)+3t^4\,dt\cr +&=-2\int_0^1 (3t^4+2t^3+4t^2+2t+2)\,dt\cr +&=-2\left({3t^5\over5}+{t^4\over2}+{4t^3\over3}+t^2+2t\right)\bigg|_0^1\cr +&=-{163\over15}\cr +}$$ + +\beginsection Green's Theorem Strikes Back! + +We have +$$P=2y-3x+5+e^y(y-1),\qquad Q=4x-3y-2+xye^y$$ +and +$${\partial P\over\partial y}=ye^y+2,\qquad{\partial Q\over\partial x}=ye^y+4$$ +Let $C_{12}$ be the path $C$ followed by the path from $(-1,0)$ back to the +starting point $(1,0)$ so that $C_{12}$ is +a closed loop. +By Green's Theorem we have +$$\int_{C_{12}} P\,dx+Q\,dy=\int\!\!\!\int_R +\left({\partial Q\over\partial x}-{\partial P\over\partial y}\right)\,dx\,dy +=2\int\!\!\!\int_R dx\,dy=2$$ +Now we have to subtract out the line integral over the path that we added to make a closed loop. +Let $C_2$ be the path from $(-1,0)$ to $(1,0)$. In addition, let +$$x=t,\qquad y=0,\qquad dx=dt,\qquad dy=0,\qquad-1\le t\le 1$$ +Then +$$\int_{C_2} P\,dx+Q\,dy=\int_{-1}^1 (-3t+4)\,dt +=\left(-{3t^2\over2}+4t\right)\bigg|_{-1}^1=8$$ +Therefore the final answer is +$$\int_C=\int_{C_{12}}-\int_{C_2}=2-8=-6$$ + + +\end