角BAO+角BFO=90度,角FBO+角BFO=90度,得角BAO=角FBO三角形ABO相似于三角形BFOOB:OF=OA:OBb:C=a:be=c/ae^2=1-b^2/a^2e^2=1-ac/a^2e^2=1-c/ae^2=1-ee=(根5-1)/2(负舍去)