令P = xln(x² + y² - 1)、Q = yln(x² + y² - 1)
∂P/∂y = 2xy/(x² + y² - 1)
∂Q/∂x = 2xy/(x² + y² - 1)
∵∂P/∂y = ∂Q/∂x
∴这个曲线积分的值与x² + y² > 1内的路径无关.