y2=2bx的焦点为(b/2,0),(c+b/2):(c-b/2)=5:3,c/2=b;
x^2/a^2+y^2/b^2=1,c^2=a^2-b^2;a^2=（c/2）^2+c^2=5c^2/4；
c/a=2√5/5