Answer:
Top-to-bottom, the boxes have this order in the proof: 1, 7, 4, 3, 9, 8, 5, 2, 6.
Step-by-step explanation:
The basic idea is to use the Pythagorean theorem to write two expressions for the length of altitude BD, also called "k", then equate them and simplify the result. This leaves an expression for DC, also called "x", which is replaced by a cosine expression to complete the proof.
Finally, the variations involving other combinations of sides and angles are suggested as being provable in the same way.
