\(\mathtt{\frac{\color{blue}{BA}}{\color{red}{DA}} = \frac{\color{purple}{CA}}{\color{orange}{EA}}}\), so \(\mathtt{\frac{\color{blue}{a + b}}{\color{red}{b}} = \frac{\color{purple}{c + d}}{\color{orange}{d}}}\).
Rewrite this as \(\mathtt{\frac{\color{red}{b}}{\color{red}{b}} + \frac{a}{\color{red}{b}} = \frac{\color{orange}{d}}{\color{orange}{d}} + \frac{c}{\color{orange}{d}}}\).