In the below article at maa site, there is geometrical proof for a generalization of the Pythagorean Theorem.
The problem given is :- Given two right triangles show that cc' = aa' + bb'.
I request the reason for the equality of the fractions $\frac{x}{b} = \frac{b'}{a}$ chosen as following some law of congruency, which is not given.
Edit As an additional request, if possible please help me identify what type of journal, is this page part of.