x^2y^2z^2/(x^2+y^2+z^2)=1/(1/y^2z^2+1/z^2x^2+1/x^2y^2)x，y，z趋向于0，x^2y^2，y^2z^2，z^2x^2均趋向于01/y^2z^2，1/z^2x^2，1/x^2y^2均趋向于正无穷属于常数/∞型limx^2y^2z^2/(x^2+y^2+z^2)=0
