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I'm attempting to use Green's Theorem to express the area of a region in the complex plane in terms of a contour integral, but I'm a little confused as to how this works. I have a simple closed curve $\gamma$ with interior $D$, and I believe I'm supposed to get $$\mathrm{Area}(D)=\frac{1}{2i} \oint_\gamma \overline{z} \,dz.$$ Can anyone help me justify this?

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$$\int_\gamma \bar{z}dz = \int_\gamma (x - iy)(dx + idy) = \int_\gamma (xdx + ydy) + i \int_\gamma( xdy - ydx)$$now hit this with stokes:$$ = \int_D d(xdx + ydy) + i\int_D d(xdy - ydx) = i \int_D 2 dx \wedge dy$$

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from $\int_D{d(xdx+ydy)} +i\int_D{d(xdy-ydx)} = i\int_D{2dxdy}$ , how you get this equality ? –  hong wai Dec 22 '12 at 14:44
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