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lemma proved tactic proof

phi_sixth_eq

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formal statement (Lean)

 169lemma phi_sixth_eq : phi^6 = 8 * phi + 5 := by

proof body

Tactic-mode proof.

 170  calc phi^6 = phi * phi^5 := by ring
 171    _ = phi * (5 * phi + 3) := by rw [phi_fifth_eq]
 172    _ = 5 * phi^2 + 3 * phi := by ring
 173    _ = 5 * (phi + 1) + 3 * phi := by rw [phi_sq_eq]
 174    _ = 8 * phi + 5 := by ring
 175
 176/-- Key identity: φ⁷ = 13φ + 8 (Fibonacci recurrence). -/

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