What is $|AC|^2 + |BD|^2= \ ?$
I asked one of my classmates for the solution, he said:
$$|AC|^2+|BD|^2+4 \cdot3^2 = 7^2 +4^2 +5^2+6^2$$
He didn't tell me how he obtained it, either because he wouldn't want to tell me (I remotely know him) or he just memorized the formula.
Provided that this is a high school geometry problem, can you show me how above equation is obtained in simple terms?