B1 (b;0), B2 (-b;0)
\( \Rightarrow {B_2}{F_1} = {B_2}{F_2} = \sqrt {{b^2} + {c^2}} \)
Do B2 thuộc elip nên:
\(\eqalign{
& {B_2}{F_1} + {B_2}{F_2} = 2a \Rightarrow 2\sqrt {{b^2} + {c^2}} = 2a \cr
& \Rightarrow {b^2} + {c^2} = {a^2} \Leftrightarrow {b^2} = {a^2} - {c^2} \cr} \)
