Some insight to better understand this equation: The distance from the center X to BC is $\frac{AX \times BC}{|BC|} = \frac{L_{BC}(x, y)}{|BC|}$ and the distance from A to BC is $\frac{L_{BC}(x_A, y_A)}{|BC|}$. Divide one distance by another and then we get $\alpha = \frac{L_{BC}(x, y)}{L_{BC}(x_A, y_A)}$.