Four arbitrary points $$\,\,\,\left( {{x_1},{y_1}} \right),\,\,\left( {{x_2},{y_2}} \right),\,\,\left( {{x_3},{y_3}} \right),\,\,\left( {{x_4},{y_4}} \right),\,\,\,\,$$ are given in the $$xy$$ $$-$$ plane using the method of least squares. If regression of $$y$$ upon $$x$$ gives the fitted line $$y=ax+b;$$ and regression of $$x$$ upon $$y$$ gives the fitted line $$x=cy+d,$$ then