∀ {𝕜 : Type u_2} {E : Type u_3} [inst : Field 𝕜] [inst_1 : LinearOrder 𝕜] [IsStrictOrderedRing 𝕜]
  [inst_3 : AddCommGroup E] [inst_4 : Module 𝕜 E] (a b c : E),
  convexJoin 𝕜 {a} (segment 𝕜 b c) = (convexHull 𝕜) {a, b, c}