Four generalizations of the Pythagorean theorem

Here are four theorems that generalize the Pythagorean theorem. Follow the links for more details regarding each equation.

1. Theorem by Apollonius for general triangles.

2. Edsgar Dijkstra's extension of the Pythagorean theorem for general triangles.

3. A generalization of the Pythagorean theorem to tetrahedra.

4. A unified Pythagorean theorem that covers spherical, plane, and hyperbolic geometry.

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