Question #342067

Use Green’s Theorem to evaluate






∮C(x − 2y2) dx + (y4 + 2xy) dy where C consists of the line segment






from (0, 2) to (0, 4), followed by the curve with parametric equations x = 4 cos t, y = 4 sin t from (0, 4) to (−2, 2√3), then the line segment from (−2, 2√3) to (−1, √3), and finally the curve with parametric equations x = 2 sin t, y = 2 cos t from (−1, √3) to (0, 2).






1
Expert's answer
2022-05-18T12:14:21-0400

ANSWER C(x2y2)dx+(y4+2xy)dy=283\oint_{C} (x-2y^{2}) dx+(y^{4}+2xy) dy = \frac{28}{3}

EXPLANATION

CC is the positively oriented , piecewise smooth , simple closed curve . Let DD is the region bounded by C.C. Denote L(x,y)=x2y2,M(x,y)=y4+2xyL(x,y)=x-2y^{2}, M(x,y)=y^{4}+2xy , then Ly=4y,Mx=2y\frac{\partial L}{\partial y}=-4y, \frac{\partial M}{\partial x}=2y . Since all functions are continuous in R2\R^{2} , by Green's theorem we have C(Ldx+Mdy)=D(MxLy)dxdy\oint_{C} \left ( Ldx+Mdy \right )=\iint_{D} \left ( \frac{\partial M}{\partial x}-\frac{\partial L }{\partial y} \right )dxdy , where the path of integration along CC is counterclockwise. Therefore,

C(x2y2)dx+(y4+2xy)dy=D(2y+4y)dxdy=6Dydxdy\oint_{C} (x-2y^{2}) dx+(y^{4}+2xy) dy =\iint_{D} ( 2y+4y)dxdy=6\iint_{D} y dxdy .



To calculate the integral we replace the Cartesian coordinates with polar coordinates: x=rcost,y=rsint.x=r\cos t , y=r\sin t. So,Dydxdy=Δrrsintdrdt\iint_{D} y dxdy=\iint_{Δ} r\cdot r \sin t dr dt , where

Δ={(t,r):π2t2π3,2r4}Δ= \left\{ \left( t,r \right) :\frac { \pi }{ 2 } \le t\le \frac { 2\pi }{ 3 } ,\quad 2\le r\le 4 \right\} .

Δrrsintdrdt=(π22π3sintdt)(24r2dr)=\iint_{Δ} r\cdot r \sin t dr dt= \left( \int _{ \frac { \pi }{ 2 } }^{ \frac { 2\pi }{ 3 } }{ \sin { t } dt } \right) \cdot \left( \int _{ 2 }^{ 4 }{ { r }^{ 2 }dr } \right) ==[cost]π22π3[r33]24=(cos2π3)(6483)=12563=283= \left [-\cos t \right ]_{\frac{\pi}{2}}^{\frac{2\pi}{3}} \cdot\left [ \frac{r^{3} }{3} \right ]_{2}^{4}=\left ( -\cos \frac{2\pi}{3} \right )\left ( \frac{64-8}{3} \right )=\frac{1}{2}\cdot\frac{56}{3}=\frac{28}{3}



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