ABC is the missing corner. On AB, randomly choose a point P. The goal is to subsequently identify point R on AC, such that rectangle APSR's area is equal to that of ABC. It is feasible. For example, draw line PC, then draw BQ in parallel to PC. It can be proven that area(ABC)=area(APQ). Find the middle point of AQ, mark it as R. So area(APSR)=area(ABC).
Now, imagine ABC is reshaped into APSR. This is possible because they are of equal area. This transforms the paper into two rectangles: the lower left small one, and the big right one. Connect the center of these two rectangles O1 O2. Cut along O1O2.
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