15.5 Assess Your Understanding

Concepts and Vocabulary

Question

Green’s Theorem relates the value of a(n) _____ integral along a simple closed piecewise smooth curve \(C\) to the value of a(n) _____ integral over the simply connected closed region \(R\) enclosed by \(C\).

Question

If the assumptions of Green’s Theorem are met, then \(\oint_{C}(P(x,y) \,dx+Q(x,y) \,dy)=\iint\limits_{R}\) ( _____) \(dx\,dy\).

Question

True or False The symbol \(\oint_C\) indicates that a simple closed curve \(C\) is to be traversed in a clockwise direction.

Question

If \(C\) is a piecewise-smooth, simple closed curve, then \( \dfrac{1}{2}\oint_{C}(-y\,dx+x\,dy)\) equals the _____ of the closed simply connected region \(R\) enclosed by \(C\).

Skill Building

In Problems 5–10, use Green’s Theorem to find each line integral. In each case, assume that \(C\) is the perimeter of the rectangle with vertices (0, 0), (4, 0), (4, 3), (0, 3), and that the orientation of \(C\) is counterclockwise.

Question

\(\oint_{C}y\,dx\)

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\(\oint_{C}x\,dy\)

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\(\oint_{C}[xy\,dx+(x+y)\,dy]\)

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\(\oint_{C}(3y\,dx-2x\,dy)\)

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\(\oint_{C}(xy^{2}\,dx+x^{2}y\,dy)\)

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\(\oint_{C}\left[ y\sin (xy) \,dx+x\sin (xy) \,dy\right] \)

In Problems 11–14, use Green’s Theorem to find each line integral. In each problem, assume that \(C\) is the unit circle \( x^{2}+y^{2}={1}\), and that the orientation of \(C\) is counterclockwise.

Question

\(\oint_{C}(-x^{2}y\,dx+y^{2}x\,dy)\)

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\(\oint_{C}[y(x^{2}+y^{2})\,dx-x(x^{2}+y^{2})\,dy]\)

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\(\oint_{C}[(x^{2}-y^{3})\,dx+(x^{2}+y^{2})\,dy]\)

Question

\(\oint_{C}[(xy^{3}+\sin x)\,dx+(x^{2}y^{2}+4x)\,dy]\)

In Problems 15–20, use Green’s Theorem to find each line integral. In each problem, \(C\) is to be traversed in the counterclockwise direction.

Question

\(\oint_{C}[(x^{2}+y)\,dx+(x-y^{2})\,dy]\), where \(C\) is the boundary of the region \(R\) bounded by \(y=x^{3/2}\), the \(x\)-axis, and \(x=1\)

Question

\(\oint_{C}[(4x^{2}-8y^{2})\,dx+(y-6xy)\,dy]\), where \(C\) is the boundary of the region \(R\) bounded by \(y=\sqrt{x}\) and \(y=x^{2}\)

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\(\oint_{C}[(x^{3}-x^{2}y)\,dx+xy^{2}\,dy]\), where \(C\) is the boundary of the region \(R\) bounded by \(y=x^{2}\) and \(x=y^{2}\)

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\(\oint_{C}[(x^{2}-y^{2})\,dx+xy\,dy]\), where \(C\) is the boundary of the region \(R\) bounded by \(x=y^{2}\) and \(x=1\)

Question

\(\oint_{C}\left[ \dfrac{1}{y}\,dx+\dfrac{1}{x}\,dy\right] \), where \(C\) is the boundary of the region \(R\) bounded by \(y=1\), \(x=9\), and \(y= \sqrt{x}\)

1015

Question

\(\oint_{C}(x^{2}y\,dx-y^{2}x\,dy)\), where \(C\) is the boundary of the region \(R\) bounded by \(y=\sqrt{a^{2}-x^{2}}\) and \(y=0\)

In Problems 21–24, find \(\oint_{\!\!\!\!C}\dfrac{y\,dx-x\,dy}{ x^{2}+y^{2}}\) about the indicated curve \(C\).

Question

\(C\): \(x^{2}+y^{2}=4\)

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\(C\): \((x-2)^{2}+(y+3)^{2}=1\)

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\(C\): \(x^{2}+4y^{2}=4\)

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\(C\): \(3x^{2}+6y^{2}=4\)

AreaIn Problems 25–32, use Green’s Theorem to find the area of each region.

Question

Bounded by \(y=x^{2}\) and \(y=x+2\)

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Bounded by \(y=x^{2}-1\) and \(y=0\)

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Bounded by \(y=x^{3}\) and \(y=x^{2}\)

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Bounded by \(y=2\sqrt{x}\) and \(y=\dfrac{x^{2}}{4}\)

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Bounded by \(y=\dfrac{1}{\sqrt{x-1}}\), \(y=0\), \(x=2\), and \(x=5\)

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Bounded by \(y=x^{3/2}\) and \(y=x\)

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Bounded by the line \(x+y=3\) and the hyperbola \(xy=2\)

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Bounded by the hyperbola \(xy=\sqrt{3}\) and the circle \(x^{2}+y^{2}=4\), in the first quadrant only

Applications and Extensions

Question

Area Find the area under one arch of the cycloid \(\mathbf{r} (t)=\left[ 2\pi t-\sin (2\pi t) \right] \,\mathbf{i}+\left[ 1-\cos (2\pi t) \right] \,\mathbf{j},\) \(0\leq t\leq 1\).

Question

Area Find the area enclosed by the hypocycloid \(x^{2/3}+y^{2/3}=1\).(Hint: Use the parametric equations \(x=\cos ^{3}t \), \(y=\sin ^{3}t\).)

Question

A folium of Descartes is given by the parametric equations \(x(t) =\dfrac{6t}{1+t^{3}},\) \(y(t) =\dfrac{ 6t^{2}}{1+t^{3}}.\) Use Green’s Theorem to show that the area \(A\) enclosed by the loop of this folium of Descartes is \(6\).

In Problems 36–40, use the formulas from Problem 52 to find the center of mass of each region.

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The region bounded by the \(x\)-axis and the semi-circle \(y=\sqrt{4-x^{2}}\)

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The region bounded by the parabola \(y=x^{2}\) and the line \(y=1\)

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The region bounded by the triangle with vertices at \((0,0) ,\) \((1,0) ,\) and \(( 0,1)\)

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The region bounded by the trapezoid with vertices at \((0,0)\), \((1,0) ,\) \(( 1,2) ,\) and \((0,1)\)

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The region bounded by the coordinate axes and the hypocycloid \(x=a\cos ^{3}t\), \(y=a\sin ^{3}t\), \(0\leq t\leq \dfrac{\pi }{2}\)

Question

Work  Use Green’s Theorem to find the work done by the variable force \(\mathbf{F}(x,y) =x\mathbf{i}+y^{2}\mathbf{j}\) in moving an object along the curve \(x(t) =t\), \(y(t) =t^{2},\) \(0\leq t\leq 4,\) and then returning to the point \((0,0) \) along the line \(x(t) =t\), \(y(t) =4t\).

Question

Work  Use Green’s Theorem to find the work done by the variable force \(\mathbf{F}(x,y) =\left( e^{x}+2y\right) \mathbf{i}+\left( e^{y}-x\right) \mathbf{j}\) in moving an object along the closed path \(x^{2}+y^{2}=9\).

Question

Find \[ \oint_{\!\!\!C} \left\{ [\ln(1+x^2) -x^2\cos y]\,dx + \left(\dfrac{x^3}{3}\cos y +e^{\cos y} \right)dy \right\} , \] where \(C\) is the boundary of the region enclosed by \(y=x^3\), \(x=1\), and the \(x\)-axis.

Question

Find \(\oint_{C} [y^3\,dx-x^3\,dy] \), where \(C\) is the boundary of the multiply-connected region \(1 \leq x^2 + y^2 \leq 9\) and \(C\) is traversed in the counter-clockwise direction.

In Problems 45–48, explain why each line integral is equal to 0.

Question

\(\oint_{C}\big(xe^{x^{2}+y^{2}}\,dx+ye^{x^{2}+y^{2}}\,dy\big)\), where \(C\) is any piecewise-smooth, simple closed curve in the plane

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\(\oint_{C}[e^{xy}(xy+1)\,dx+x^{2}e^{xy}\,dy]\), where \(C\) is any piecewise-smooth, simple closed curve in the plane

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\(\oint_{C}(e^{x}\sin y\,dx+e^{x}\cos y\,dy)\), where \(C\) is any piecewise-smooth, simple closed curve in the plane

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\(\oint_{C}\left[ \dfrac{x\,dx}{(x^{2}+y^{2})^{1/2}}+\dfrac{ y\,dy}{(x^{2}+y^{2})^{1/2}}\right] \), where \(C\) is any piecewise-smooth, simple closed curve not containing the origin in its interior

Question

Green’s Theorem  Complete the proof of Green’s Theorem for \(x\) -simple and \(y\)-simple regions by showing \(\oint_{C}Q(x,y)\,dy=\iint \limits_{ R}\dfrac{\partial Q}{\partial x}\,dx\,dy\).

Question

Use Green’s Theorem for finding the area \(A\) of the region \(R\) enclosed by simple closed piecewise smooth curve \(C\) to show that \(A=\oint_{C}x\,dy\) and \(A=\oint_{C}(-y)\,dx\).

Question

Consider the integral \(\int_{C}\dfrac{y\,dx-x\,dy}{ (x^{2}+y^{2})^{3/2}}\). For which simple closed, piecewise-smooth curves \(C\) can Green’s Theorem be applied to transform this line integral to a double integral?

Challenge Problems

Question

Center of Mass A homogeneous lamina occupies a region in the \( xy\)-plane with boundary \(C\) (oriented counterclockwise). Show that its center of mass has the coordinates given below, where \(A\) is the area of the region. \[ \bar{x}=\dfrac{1}{2A}\oint\nolimits_{\!\!C}x^{2}\,dy\qquad \bar{y}=\dfrac{-1}{2A} \oint\nolimits_{\!\!C}y^{2}\,dx \]

Question

Moment of Inertia Use Problem 52 to show that the moment of inertia about the origin is \[ I_{o}=\dfrac{1}{3}\int_{C}(x^{3}\,dy-y^{3}\,dx) \] (Assume that the mass density of the lamina is \(1\).)

Question

Let \(R\) be a simply connected closed region in the \(xy\) -plane, and let \(R′ \) be a simply connected closed region in the \(st\) -plane. Suppose \(x=x(s,t)\) and \(y=y(s,t)\) are differentiable and that, as \((s,t)\) is allowed to vary throughout \(R′\), \((x,y)\) varies throughout \(R \) in a one-to-one correspondence. Suppose further that \(R\) has boundary \(C\) , which is a piecewise-smooth closed curve, and \(R′ \) has boundary \(C′ \), which is a piecewise-smooth closed curve. Finally, suppose that as \((s,t)\) moves along \(C′ \), \((x(s,t),y(s,t))\) moves along \(C\). Show that \[ \iint\limits_{R}\,dx\,dy=\iint\limits_{R^{\prime }} (x_{s}y_{t}-x_{t}y_{s}) \,ds\,dt \]

[Hint: Using \(\iint\limits_{R}\,dx\,dy=\dfrac{1}{2}\oint_{C}(x\,dy-y\,dx)\), write the integral on the right in terms of \(s\) and \(t\). Then apply Green’s Theorem to the resulting integral, which will have the form \(\oint_{C’ }(P\,ds+Q\,dt)\).]

1016

Question

Transformation of a Double Integral Let \(\phi (x,y)\) be continuous and let \(P(x,y)=-\int \phi (x,y)\,dy\) and \(Q(x,y)=\int \phi (x,y)\,dx\). Generalize the result of Problem 54 using \(P\) and \(Q\) to show that \[ \iint\limits_{R}\phi (x,y)\,dx\,dy=\iint\limits_{R^{\prime }}\phi (x(s,t),y(s,t))(x_{s}y_{t}-x_{t}y_{s}) \,ds\,dt \] Go through a similar process as in Problem 54, using Green’s Theorem on the integral \(\oint_{C}P\,dx+Q\,dy\).