Missing Square

[theblindferrengi]

This was an easy one.

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The figures are neither congruent nor triangles. The triangle that is made from the three acute angles along the perimiter of the figure should have a hypotenuse with a slope of 5/13. The red triangle has a hypotenuse with a slope of 3/8, and the blue triangle has a slope of 2/5. The small triangles should have slope congruent to eachother and the triangle-ish figure they make, since they dont there is an obtuse angle along the hypotenuse of the triangle-like large figure, making it an irregular tetragon.

When you switch the red and blue triangles, it changes the concave obtuse angle into a convex obtuse angle, therefore changing the area. The missing aquare comes from the sliver that is changed along the hypotenuse.

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Edit: Damn Adrian, cut me off.