Let formula_229 be the point on the line formula_230 with the distance formula_4 to the focus formula_26, formula_20 is the semi-major axis of the ellipse. Let line formula_234 be the bisector of the supplementary angle to the angle between the lines formula_228. In order to prove that formula_234 is the tangent line at point formula_14, one checks that any point formula_238 on line formula_234 which is different from formula_14 cannot be on the ellipse. Hence formula_234 has only point formula_14 in common with the ellipse and is, therefore, the tangent at point formula_14.