Legendre's theorem on spherical triangles

In geometry, Legendre's theorem on spherical triangles, named after Adrien-Marie Legendre, is stated as follows: Let ABC be a spherical triangle on the unit sphere with small sides a, b, c. Let A'B'C' be the planar triangle with the same sides.

Source: Wikipedia — Legendre's theorem on spherical triangles (CC BY-SA 4.0)