Answer
We consider a rectangle. where the sides are a, b, c, and d and where the diagonals are A and B. Thus, we obtain:
$A^2 = a^2 + b^2 $
And:
$B^2 = c^2 + d^2 $
Adding these gives:
$ A^2 + B^2 = a^2 + b^2 +c^2 + d^2$
Thus, the proof is completed.
