What proof the author of these two diagrams suggests?
We thought that it was in the animation of Example 1.

Actually, the authors on Page 027 of Book [C] suggest the proof
in our animated diagram. One color is enough!

According to the authors, the solution is in the Chou Pei Suan Ching.
However, I am not sure that the diagram in Example 1 in the Chou Pei Suan Ching,
did suggest this solution. It might be credited to the authors of book [C]: Alsina and Nelsen.

Independently, Professor D. S. Wise of Indiana University came up to this solution also.
Please, click the above frame to see the diagram in animation