c=√(a^2-b^2)=√(16-9)=√7
左焦点F1(-√7,0)
将x=-√7代入x^2/16+y^2/9=1,7/16+y^2/9=1,y=±9/4,即A、B坐标（-√7,±9/4)
|AF2| = √[(xF2-XA)^2+(yF2-yA)^2] = √[(√7+√7)^2+(0±9/4)^2] = √(529/16) = 23/4