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lemma proved tactic proof

phi_pow_7_eq

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formal statement (Lean)

  57private lemma phi_pow_7_eq : phi ^ 7 = 13 * phi + 8 := by

proof body

Tactic-mode proof.

  58  have h6 := phi_sixth_eq
  59  have hsq := phi_sq_eq
  60  calc phi ^ 7 = phi * phi ^ 6 := by ring
  61    _ = phi * (8 * phi + 5) := by rw [h6]
  62    _ = 8 * phi ^ 2 + 5 * phi := by ring
  63    _ = 8 * (phi + 1) + 5 * phi := by rw [hsq]
  64    _ = 13 * phi + 8 := by ring
  65

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