Inside the box II

A square contains four other squares. Prove that the red line segment is parallel to its vertical sides.

Scroll down for a solution to this problem.


Solution by chu.

Explanation by Marshall W. Buck: Say the whole square is 1 by 1. Vertical height of any colored square will equal its horizontal width. If yellow’s is x then green is 1-x then blue is x and purple is 1-x again. Thus the red segment is vertical.

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