设沿着 y = kx 这条直线趋近于原点,则有：lim (x^2 *y^2)/[x^2 + y^2 + (x - y)^2]=lim (k^2 *x^4) /[x^2 + k^2 *x^2 + (k -1)^2 * x^2]=lim k^2 *x^2 /[1 + k^2 + (k -1)^2 ]=lim k^2 * 0^2 /[1 + k^2 + (k -1)^2 ]...