Áp dụng $B-C-S$ dạng Engel ta được:
$\begin{array}{l} \dfrac{x}{{1 - x}} + \dfrac{y}{{1 - y}} = 1\\ \Leftrightarrow \dfrac{{1 + x - 1}}{{1 - x}} + \dfrac{{1 - y - 1}}{{1 - y}} = 1\\ \Leftrightarrow - 1 + \dfrac{1}{{1 - x}} - 1 + \dfrac{1}{{1 - y}} = 1\\ \Leftrightarrow \dfrac{1}{{1 - x}} + \dfrac{1}{{1 - y}} = 3\\ \Leftrightarrow 3 = \dfrac{1}{{1 - x}} + \dfrac{1}{{1 - y}} \ge \dfrac{4}{{2 - \left( {x + y} \right)}}\left( {B - C - S} \right)\\ \Leftrightarrow 6 - 3\left( {x + y} \right) \ge 4\\ \Leftrightarrow 3\left( {x + y} \right) \le 2 \Leftrightarrow x + y \le \dfrac{2}{3} \end{array}$
