How do you find out the missing piece to an area model?

Answer 1

Usually the two triangle pieces have different pitches. So by swapping them it makes the total triangle area different.

Red triangle pitch: atan(3/8) = 20.56 deg

Blue triangle pitch: atan(2/5) = 21.80 deg

When the blue is on the top, the total triangle bends inwards slightly, and when its at the bottom, the total bends outwards. Thus changing the area of the triangle and making room for an empty block.