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lemma proved tactic proof

phi_tenth_eq

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formal statement (Lean)

 201lemma phi_tenth_eq : phi^10 = 55 * phi + 34 := by

proof body

Tactic-mode proof.

 202  calc phi^10 = phi * phi^9 := by ring
 203    _ = phi * (34 * phi + 21) := by rw [phi_ninth_eq]
 204    _ = 34 * phi^2 + 21 * phi := by ring
 205    _ = 34 * (phi + 1) + 21 * phi := by rw [phi_sq_eq]
 206    _ = 55 * phi + 34 := by ring
 207
 208/-- Key identity: φ¹¹ = 89φ + 55 (Fibonacci recurrence). -/

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