The key to this puzzle is that the two polygons of 13x5 are not triangles, so it is not easy to detect the slope difference of the hypotenuse of the red triangle and the blue triangle by visual inspection. So I mistakenly thought that the two combined figures were triangles.

Four figures (yellow, red, blue and green) occupy 32 unit areas, but the total triangles outside are 13 and 5, totaling 32.5 units. The aspect ratio of the blue triangle is 5:2, and the aspect ratio of the red triangle is 8:3, and these two are not the same aspect ratio. So in each figure, the hypotenuse after addition is actually shortened.

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