ANSWER ∮C(x−2y2)dx+(y4+2xy)dy=328
EXPLANATION
C is the positively oriented , piecewise smooth , simple closed curve . Let D is the region bounded by C. Denote L(x,y)=x−2y2,M(x,y)=y4+2xy , then ∂y∂L=−4y,∂x∂M=2y . Since all functions are continuous in R2 , by Green's theorem we have ∮C(Ldx+Mdy)=∬D(∂x∂M−∂y∂L)dxdy , where the path of integration along C is counterclockwise. Therefore,
∮C(x−2y2)dx+(y4+2xy)dy=∬D(2y+4y)dxdy=6∬Dydxdy .
To calculate the integral we replace the Cartesian coordinates with polar coordinates: x=rcost,y=rsint. So,∬Dydxdy=∬Δr⋅rsintdrdt , where
Δ={(t,r):2π≤t≤32π,2≤r≤4} .
∬Δr⋅rsintdrdt=(∫2π32πsintdt)⋅(∫24r2dr)==[−cost]2π32π⋅[3r3]24=(−cos32π)(364−8)=21⋅356=328
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