z = ln[(x-y)/(x+y)] = ln|x-y| - ln|x+y| ∂z/∂x = 1/(x-y) - 1/(x+y) = 2y/(x^2-y^2)∂z/∂y = -1/(x-y) - 1/(x+y) = -2x/(x^2-y^2)dz = 2(ydx-xdy)/(x^2-y^2)
