Green's theorem relates a line integral around a simple closed curve C to a double integral over the plane region D bounded by C. It states that a line integral around the boundary of a plane region D can be calculated as the double integral over region D. The theorem allows contour integrals to be transformed into double integrals over the interior region, and plays a key role in reciprocity in electromagnetism. The document provides the statement of Green's theorem and gives a proof of the theorem by dividing the bounding curve C into two parts and considering the equations of the curves.