Hello everyone, This is the link to the problem Problem

I saw it come up while neal was explaining Educational Round 99 E and I visited this question because I was intrigued by the Geometry involved. Can anyone explain me the case when both the rectangles are square, then the area of Intersection is = (Area of the unrotated rectangle — 4*(area of any one of the triangles formed after intersection)). This is the tutorial I want to understand the proof behind as to why the area of all the triangles are equal? I wanted to understand this. Help me. Thanks in Advance