For any rectangle, the sum of the squares of the distances
from any point P to two opposite corners is equal to the sum
of the squares of the distances from that point to the two
other corners (so, above, a² + c² = b² + d²).
This remains true whether the point is inside or outside the rectangle,
on a side or a corner, or even outside the plane.
credit: Greg Ross
http://www.futilitycloset.com/2013/04/07/balance-5/
