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Use Green's theorem to find the work done by the force F(x,y)=x(x+y)+x2)\mathbf{F}(x,y)=x(x+y)+x^{2}) in moving a particle from the origin along the xx-axis to (5,0)(5,0), then along the line segment to (0,5)(0,5), and then back to the origin along the yy-axis.

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Q. Use Green's theorem to find the work done by the force F(x,y)=x(x+y)+x2)\mathbf{F}(x,y)=x(x+y)+x^{2}) in moving a particle from the origin along the xx-axis to (5,0)(5,0), then along the line segment to (0,5)(0,5), and then back to the origin along the yy-axis.
  1. Set up problem using Green's theorem: Understand and set up the problem using Green's theorem. Green's theorem relates a line integral around a simple closed curve CC and a double integral over the plane region DD bounded by CC. We need to convert the force field into a form suitable for applying Green's theorem. F(x,y)=x(x+y)+x2F(x, y) = x(x + y) + x^2 can be rewritten as P(x,y)=x(x+y)P(x, y) = x(x + y) and Q(x,y)=x2Q(x, y) = x^2.
  2. Apply Green's theorem: Apply Green's theorem. Green's theorem states that the line integral of FF around CC is equal to the double integral over DD of (QxPy)dA\left(\frac{\partial Q}{\partial x} - \frac{\partial P}{\partial y}\right) dA. Calculate Qx\frac{\partial Q}{\partial x} and Py\frac{\partial P}{\partial y}: Qx=2x\frac{\partial Q}{\partial x} = 2x, Py=x\frac{\partial P}{\partial y} = x. So, QxPy=2xx=x\frac{\partial Q}{\partial x} - \frac{\partial P}{\partial y} = 2x - x = x.
  3. Set up double integral: Set up the double integral. The region DD is a right triangle with vertices at (0,0)(0,0), (5,0)(5,0), and (0,5)(0,5). The limits of integration are from x=0x = 0 to x=5x = 5 and for each xx, yy goes from 00 to 5x5 - x. Set up the integral: (0,0)(0,0)00.
  4. Calculate double integral: Calculate the double integral.\newlineFirst, integrate with respect to yy:\newliney=05xxdy=x[y]05x=x(5x)\int_{y=0}^{5-x} x \, dy = x[y]_{0}^{5-x} = x(5-x).\newlineNow, integrate with respect to xx:\newlinex=05x(5x)dx=x=05(5xx2)dx\int_{x=0}^{5} x(5-x) \, dx = \int_{x=0}^{5} (5x - x^2) \, dx.
  5. Finish calculation: Finish the calculation.\newlineCalculate x=05(5xx2)dx\int_{x=0}^{5} (5x - x^2) dx:\newline= [5x22x33][\frac{5x^2}{2} - \frac{x^3}{3}] from 00 to 55\newline= (52521253)(5\cdot\frac{25}{2} - \frac{125}{3})\newline= (12521253)(\frac{125}{2} - \frac{125}{3})\newline= (37562506)(\frac{375}{6} - \frac{250}{6})\newline= \frac{125125}{66}.

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