13.已知椭圆C:x2a2+y2b2=1(a>b>0)C:\frac{x^2}{a^2}+\frac{y^2}{b^2}=1({a>b>0})C:a2x2+b2y2=1(a>b>0)的左顶点为AAA,上顶点为BBB,右焦点为FFF,若∠ABF=90∘\angle ABF=90\circ∠ABF=90∘,则椭圆CCC的离心率为((( )))