This is the theorem of Pythagore on the pseudo-euclidean plane used to sketch the Minkowski diagrams of the special theory of relativity. The fixed point carries the perpendicular kathetes. The difference of the two squares euals the square about the hypotenuse.
The geometric properties of the space-time plane not only illustrate the statements of the theory of special relativity, they can be used to logically derive its kinematics and to unambiguously interpret its statements, which are misunderstood too often.

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