21.已知双曲线E:x2a2−y2b2=1(a>0,b>0)E:\frac{x^2}{a^2}-\frac{y^2}{b^2}=1(a>0,b>0)E:a2x2−b2y2=1(a>0,b>0)的焦距为4,若x2−y2=a2x^{2}-y^{2}=a^{2}x2−y2=a2,则EEE的方程为 ___x22−y22=1\frac{x^2}{2}-\frac{y^2}{2}=12x2−2y2=1___.