AF=2OB
即有a+c=2b
c^2=a^2-b^2=a^2-(a+c)^2/4
4c^2=4a^2-(a^2+2ac+c^2)
3a^2-2ac-5c^2=0
3-2c/a-5c^2/a^2=0
即5e^2+2e-3=0
(5e-3)(e+1)=0
e=3/5
AF=2OB
即有a+c=2b
c^2=a^2-b^2=a^2-(a+c)^2/4
4c^2=4a^2-(a^2+2ac+c^2)
3a^2-2ac-5c^2=0
3-2c/a-5c^2/a^2=0
即5e^2+2e-3=0
(5e-3)(e+1)=0
e=3/5