y2=2bx的焦点坐标为F(b/2,0)
F1(-c,0) F2(c,0)
(c+b/2)/(c-b/2)=5/3
c=2b
a^2=b^2+c^2=5b^2
a=√5b
e=c/a=2b/√5b=(2√5)/5