设焦点在y轴上的双曲线的标准方程y^2/a^2-x^2/b^2=1.a=2根号5代入:y^2/20-x^2/b^2=1(2,-5)代入得:25/20-4/b^2=1b^2=16.即方程是y^2/20-x^2/16=1
