theorem proved term proof

`linear_cosine`

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formal statement (Lean)

  67theorem linear_cosine : optimalBondCosine 2 = -1 := by

Term-mode proof.

  68  simp only [optimalBondCosine]
  69  norm_num
  70
  71/-- Trigonal planar (n=3) has angle ≈ 120° (cos = -1/2). -/

Lean names referenced from this declaration's body.