c=1
F1(-1,0),F2(1,0)
y=x+1
4x^2+5(x+1)^2=20
9x^2+10x-15=0
xA-xB=yA-yB=8√10/9
AB^2=2(xa-xB)^2+(yA-yB)^2=1280/81
AB=16√5/9