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15.4E: Green's Theorem (Exercises)

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For the following exercises, evaluate the line integrals by applying Green’s theorem.

1. C2xydx+(x+y)dy, where C is the path from (0,0) to (1,1) along the graph of y=x3 and from (1,1) to (0,0) along the graph of y=x oriented in the counterclockwise direction

2. C2xydx+(x+y)dy, where C is the boundary of the region lying between the graphs of y=0 and y=4x2 oriented in the counterclockwise direction

Answer
C2xydx+(x+y)dy=323 units of work

3. C2arctan(yx)dx+ln(x2+y2)dy, where C is defined by x=4+2cosθ,y=4sinθ oriented in the counterclockwise direction

4. Csinxcosydx+(xy+cosxsiny)dy, where C is the boundary of the region lying between the graphs of y=x and y=x oriented in the counterclockwise direction

Answer
Csinxcosydx+(xy+cosxsiny)dy=112 units of work

5. Cxydx+(x+y)dy, where C is the boundary of the region lying between the graphs of x2+y2=1 and x2+y2=9 oriented in the counterclockwise direction

6. C(ydx+xdy), where C consists of line segment C1 from (1,0) to (1,0), followed by the semicircular arc C2 from (1,0) back to (1,0)

Answer
C(ydx+xdy)=π units of work

For the following exercises, use Green’s theorem.

7. Let C be the curve consisting of line segments from (0,0) to (1,1) to (0,1) and back to (0,0). Find the value of Cxydx+y2+1dy.

8. Evaluate line integral Cxe2xdx+(x4+2x2y2)dy, where C is the boundary of the region between circles x2+y2=1 and x2+y2=4, and is a positively oriented curve.

Answer
Cxe2xdx+(x4+2x2y2)dy=0 units of work

9. Find the counterclockwise circulation of field F(x,y)=xyˆi+y2ˆj around and over the boundary of the region enclosed by curves y=x2 and y=x in the first quadrant and oriented in the counterclockwise direction.

CNX_Calc_Figure_16_04_201.jpg

10. Evaluate Cy3dxx3y2dy, where C is the positively oriented circle of radius 2 centered at the origin.

Answer
Cy3dxx3y2dy=20π units of work

11. Evaluate Cy3dxx3dy, where C includes the two circles of radius 2 and radius 1 centered at the origin, both with positive orientation.

CNX_Calc_Figure_16_04_202.jpg

12. Calculate Cx2ydx+xy2dy, where C is a circle of radius 2 centered at the origin and oriented in the counterclockwise direction.

Answer
Cx2ydx+xy2dy=8π units of work

13. Calculate integral C2[y+xsin(y)]dx+[x2cos(y)3y2]dy along triangle C with vertices (0,0),(1,0) and (1,1), oriented counterclockwise, using Green’s theorem.

14. Evaluate integral C(x2+y2)dx+2xydy, where C is the curve that follows parabola y=x2 from (0,0),(2,4), then the line from (2,4) to (2,0), and finally the line from (2,0) to (0,0).

CNX_Calc_Figure_16_04_203.jpg

Answer
C(x2+y2)dx+2xydy=0 units of work

15. Evaluate line integral C(ysin(y)cos(y))dx+2xsin2(y)dy, where C is oriented in a counterclockwise path around the region bounded by x=1,x=2,y=4x2, and y=x2.

CNX_Calc_Figure_16_04_204.jpg

For the following exercises, use Green’s theorem to find the area.

16. Find the area between ellipse x29+y24=1 and circle x2+y2=25.

Answer
A=19πunits2

17. Find the area of the region enclosed by parametric equation

p(θ)=(cos(θ)cos2(θ))ˆi+(sin(θ)cos(θ)sin(θ))ˆj for 0θ2π.

CNX_Calc_Figure_16_04_205.jpg

18. Find the area of the region bounded by hypocycloid r(t)=cos3(t)ˆi+sin3(t)ˆj. The curve is parameterized by t[0,2π].

Answer
A=38πunits2

19. Find the area of a pentagon with vertices (0,4),(4,1),(3,0),(1,1), and (2,2).

20. Use Green’s theorem to evaluate C+(y2+x3)dx+x4dy, where C+ is the perimeter of square [0,1]×[0,1] oriented counterclockwise.

Answer
C+(y2+x3)dx+x4dy=0

21. Use Green’s theorem to prove the area of a disk with radius a is A=πa2units2.

22. Use Green’s theorem to find the area of one loop of a four-leaf rose r=3sin2θ. (Hint: xdyydx=r2dθ).

Answer
A=9π8units2

23. Use Green’s theorem to find the area under one arch of the cycloid given by the parametric equations: x=tsint,y=1cost,t0.

24. Use Green’s theorem to find the area of the region enclosed by curve

r(t)=t2ˆi+(t33t)ˆj, for 3t3.

CNX_Calc_Figure_16_04_206.jpg

Answer
A=835units2

25. [T] Evaluate Green’s theorem using a computer algebra system to evaluate the integral Cxeydx+exdy, where C is the circle given by x2+y2=4 and is oriented in the counterclockwise direction.

26. Evaluate C(x2y2xy+y2)ds, where C is the boundary of the unit square 0x1,0y1, traversed counterclockwise.

Answer
C(x2y2xy+y2)ds=3

27. Evaluate C(y+2)dx+(x1)dy(x1)2+(y+2)2, where C is any simple closed curve with an interior that does not contain point (1,2) traversed counterclockwise.

28. Evaluate Cxdx+ydyx2+y2, where C is any piecewise, smooth simple closed curve enclosing the origin, traversed counterclockwise.

Answer
Cxdx+ydyx2+y2=2π

For the following exercises, use Green’s theorem to calculate the work done by force F on a particle that is moving counterclockwise around closed path C.

29. F(x,y)=xyˆi+(x+y)ˆj,C:x2+y2=4

30. F(x,y)=(x3/23y)ˆi+(6x+5y)ˆj,C: boundary of a triangle with vertices (0,0),(5,0), and (0,5)

Answer
W=2252 units of work

31. Evaluate C(2x3y3)dx+(x3+y3)dy, where C is a unit circle oriented in the counterclockwise direction.

32. A particle starts at point (2,0), moves along the x-axis to (2,0), and then travels along semicircle y=4x2 to the starting point. Use Green’s theorem to find the work done on this particle by force field F(x,y)=xˆi+(x3+3xy2)ˆj.

Answer
W=12π units of work

33. David and Sandra are skating on a frictionless pond in the wind. David skates on the inside, going along a circle of radius 2 in a counterclockwise direction. Sandra skates once around a circle of radius 3, also in the counterclockwise direction. Suppose the force of the wind at point (x,y) is F(x,y)=(x2y+10y)ˆi+(x3+2xy2)ˆj. Use Green’s theorem to determine who does more work.

34. Use Green’s theorem to find the work done by force field F(x,y)=(3y4x)ˆi+(4xy)ˆj when an object moves once counterclockwise around ellipse 4x2+y2=4.

Answer
W=2π units of work

35. Use Green’s theorem to evaluate line integral Ce2xsin2ydx+e2xcos2ydy, where C is ellipse 9(x1)2+4(y3)2=36 oriented counterclockwise.

36. Evaluate line integral Cy2dx+x2dy, where C is the boundary of a triangle with vertices (0,0),(1,1), and (1,0), with the counterclockwise orientation.

Answer
Cy2dx+x2dy=13 units of work

37. Use Green’s theorem to evaluate line integral Ch·dr if h(x,y)=eyˆisinπxˆj, where C is a triangle with vertices (1,0),(0,1), and (1,0), traversed counterclockwise.

38. Use Green’s theorem to evaluate line integral C1+x3dx+2xydy where C is a triangle with vertices (0,0),(1,0), and (1,3) oriented clockwise.

Answer
\displaystyle \int_C \sqrt{1+x^3}\,dx+2xy\,dy=3 units of work

39. Use Green’s theorem to evaluate line integral \displaystyle \int_C x^2y\,dx−xy^2\,dy where C is a circle x^2+y^2=4 oriented counterclockwise.

40. Use Green’s theorem to evaluate line integral \displaystyle \int_C \left(3y−e^{\sin x}\right)\,dx+\left(7x+\sqrt{y^4+1}\right)\,dy where C is circle x^2+y^2=9 oriented in the counterclockwise direction.

Answer
\displaystyle \int_C \left(3y−e^{\sin x}\right)\,dx+\left(7x+\sqrt{y^4+1}\right)\,dy=36π units of work

41. Use Green’s theorem to evaluate line integral \displaystyle \int_C (3x−5y)\,dx+(x−6y)\,dy, where C is ellipse \frac{x^2}{4}+y^2=1 and is oriented in the counterclockwise direction.

A horizontal oval oriented counterclockwise with vertices at (-2,0), (0,-1), (2,0), and (0,1). The region enclosed is shaded.

42. Let C be a triangular closed curve from (0, 0) to (1, 0) to (1, 1) and finally back to (0, 0). Let \vecs F(x,y)=4y\,\mathbf{\hat i}+6x^2\,\mathbf{\hat j}. Use Green’s theorem to evaluate \displaystyle ∮_C\vecs F·d\vecs r.

Answer
\displaystyle ∮_C\vecs F·d\vecs r=2 units of work

43. Use Green’s theorem to evaluate line integral \displaystyle ∮_C y\,dx−x\,dy, where C is circle x^2+y^2=a^2 oriented in the clockwise direction.

44. Use Green’s theorem to evaluate line integral \displaystyle ∮_C (y+x)\,dx+(x+\sin y)\,dy, where C is any smooth simple closed curve joining the origin to itself oriented in the counterclockwise direction.

Answer
\displaystyle ∮_C (y+x)\,dx+(x+\sin y)\,dy=0 units of work

45. Use Green’s theorem to evaluate line integral \displaystyle ∮_C \left(y−\ln(x^2+y^2)\right)\,dx+\left(2\arctan \frac{y}{x}\right)\,dy, where C is the positively oriented circle (x−2)^2+(y−3)^2=1.

46. Use Green’s theorem to evaluate \displaystyle ∮_C xy\,dx+x^3y^3\,dy, where C is a triangle with vertices (0, 0), \,(1, 0), and (1, 2) with positive orientation.

Answer
\displaystyle ∮_C xy\,dx+x^3y^3\,dy=2221 units of work

47. Use Green’s theorem to evaluate line integral \displaystyle \int_C \sin y\,dx+x\cos y\,dy, where C is ellipse x^2+xy+y^2=1 oriented in the counterclockwise direction.

48. Let \vecs F(x,y)=\left(\cos(x^5)−13y^3\right)\,\mathbf{\hat i}+13x^3\,\mathbf{\hat j}. Find the counterclockwise circulation \displaystyle ∮_C\vecs F·d\vecs r, where C is a curve consisting of the line segment joining (−2,0) and (−1,0), half circle y=\sqrt{1−x^2}, the line segment joining (1, 0) and (2, 0), and half circle y=\sqrt{4−x^2}.

Answer
\displaystyle ∮_C\vecs F·d\vecs r=15π^4 units of work

49. Use Green’s theorem to evaluate line integral \displaystyle ∫_C \sin(x^3)\,dx+2ye^{x^2}\,dy, where C is a triangular closed curve that connects the points (0, 0), \,(2, 2), and (0, 2) counterclockwise.

50. Let C be the boundary of square 0≤x≤π,\;0≤y≤π, traversed counterclockwise. Use Green’s theorem to find \displaystyle ∫_C \sin(x+y)\,dx+\cos(x+y)\,dy.

Answer
\displaystyle \int_C\sin(x+y)\,dx+\cos(x+y)\,dy=4 units of work

51. Use Green’s theorem to evaluate line integral \displaystyle ∫_C \vecs F·d\vecs r, where \vecs F(x,y)=(y^2−x^2)\,\mathbf{\hat i}+(x^2+y^2)\,\mathbf{\hat j}, and C is a triangle bounded by y=0,\;x=3, and y=x, oriented counterclockwise.

52. Use Green’s Theorem to evaluate integral \displaystyle ∫_C \vecs F·d\vecs r, where \vecs F(x,y)=(xy^2)\,\mathbf{\hat i}+x\,\mathbf{\hat j}, and C is a unit circle oriented in the counterclockwise direction.

Answer
\displaystyle ∫_C \vecs F·d\vecs r=π units of work

53. Use Green’s theorem in a plane to evaluate line integral \displaystyle ∮_C (xy+y^2)\,dx+x^2\,dy, where C is a closed curve of a region bounded by y=x and y=x^2 oriented in the counterclockwise direction.

54. Calculate the outward flux of \vecs F(x,y)=−x\,\mathbf{\hat i}+2y\,\mathbf{\hat j} over a square with corners (±1,\,±1), where the unit normal is outward pointing and oriented in the counterclockwise direction.

Answer
\displaystyle ∮_C\vecs F·\vecs N \,ds=4

55. [T] Let C be circle x^2+y^2=4 oriented in the counterclockwise direction. Evaluate \displaystyle ∮_C \left[\left(3y−e^{\arctan x})\,dx+(7x+\sqrt{y^4+1}\right)\,dy\right] using a computer algebra system.

56. Find the flux of field \vecs F(x,y)=−x\,\mathbf{\hat i}+y\,\mathbf{\hat j} across x^2+y^2=16 oriented in the counterclockwise direction.

Answer
\displaystyle ∮_C \vecs F·\vecs N\,ds=32π

57. Let \vecs F=(y^2−x^2)\,\mathbf{\hat i}+(x^2+y^2)\,\mathbf{\hat j}, and let C be a triangle bounded by y=0, \,x=3, and y=x oriented in the counterclockwise direction. Find the outward flux of \vecs F through C.

58. [T] Let C be unit circle x^2+y^2=1 traversed once counterclockwise. Evaluate \displaystyle ∫_C \left[−y^3+\sin(xy)+xy\cos(xy)\right]\,dx+\left[x^3+x^2\cos(xy)\right]\,dy by using a computer algebra system.

Answer
\displaystyle ∫_C \left[−y^3+\sin(xy)+xy\cos(xy)\right]\,dx+\left[x^3+x^2\cos(xy)\right]\,dy=4.7124 units of work

59. [T] Find the outward flux of vector field \vecs F(x,y)=xy^2\,\mathbf{\hat i}+x^2y\,\mathbf{\hat j} across the boundary of annulus R=\big\{(x,y):1≤x^2+y^2≤4\big\}=\big\{(r,θ):1≤r≤2,\,0≤θ≤2π\big\} using a computer algebra system.

60. Consider region R bounded by parabolas y=x^2 and x=y^2. Let C be the boundary of R oriented counterclockwise. Use Green’s theorem to evaluate \displaystyle ∮_C \left(y+e^{\sqrt{x}}\right)\,dx+\left(2x+\cos(y^2)\right)\,dy.

Answer
\displaystyle ∮_C \left(y+e^{\sqrt{x}}\right)\,dx+\left(2x+\cos(y^2)\right)\,dy=13 units of work

Contributors

Gilbert Strang (MIT) and Edwin “Jed” Herman (Harvey Mudd) with many contributing authors. This content by OpenStax is licensed with a CC-BY-SA-NC 4.0 license. Download for free at http://cnx.org.


This page titled 15.4E: Green's Theorem (Exercises) is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by OpenStax.

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