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7 votes

Triangle area on surfaces of constant curvature

All the "elementary derivations of that type" are cheating (it may look nice but it proves nothing). The only elementary way to introduce area is adding it as an axiom (which is already kind of chea …
Anton Petrunin's user avatar
10 votes
Accepted

Convex cones and self-duality

For convex figure $\Sigma$ in $\mathbb S^2$, the isoperimetrical inequality should look like $$\left(\frac{\mathop{\rm perim}\Sigma}{2\cdot\pi}\right)^2+\left(1-\frac{\mathop{\rm area}\Sigma}{2\cdot\p …
Anton Petrunin's user avatar