theorem proved term proof

`tetrahedral_cosine`

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formal statement (Lean)

  77theorem tetrahedral_cosine : optimalBondCosine 4 = -1/3 := by

Term-mode proof.

  78  simp only [optimalBondCosine]
  79  norm_num
  80
  81/-- Octahedral (n=6) gives cos = -1/5 from formula, but real octahedral uses 90°. -/

Lean names referenced from this declaration's body.