The areas of the four sections are:
- Blue = ½ x 8 x 3 = 12
- Green = ½ x 5 x 2 = 5
- Orange = 7
- Pink = 8
A total of 12 + 5 + 7 + 8 = 32 squares.
However, the area of the overall triangle appears to be
- ½ x 13 x 5 = 32.5 squares
Clearly something unusual is going on.
When the pieces are moved around, again they do not form a true triangle, this time what appears to be the diagonal of the larger triangle is actually bent outwards slightly.
The difference between the first one bent inwards, and the second one bent outwards measures exactly 1 square, which is the missing square.