Triangle ratio (Analogous triangles)
Move only the "A" dot. If you move A you will see that the DCE area goes from nearly 0 to nearly infinity. Area of DCE function of A is continuous (nos "gaps" or "jumps" in values), therefore, it is possible to place point A as such DCE=3 (or whatever you want). By design, BCE and CDB are 90° angles. Have fun.