This can be established by dividing the area into strips of infinitesimal width \(dy\) parallel to the x-axis, integrating with respect to \(x\) within a strip, to give \(\int_{strip}(\partial Q/\part...This can be established by dividing the area into strips of infinitesimal width \(dy\) parallel to the x-axis, integrating with respect to \(x\) within a strip, to give \(\int_{strip}(\partial Q/\partial x)dx=Q(x_2,y_1)-Q(x_1,y_1)\), the co-ordinates of the points on the contour at the ends of the strip, then adding the contributions from all the parallel strips just gives the integral around the contour.)
