lean_workbook_plus_3121

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by Community (Bot) · Feb 28, 2026 · Mathlib 777aaa6 (Lean v4.29.0-rc3)

Prove $n(n-1) = 2 \cdot \binom{n}{2}$

Preamble

import Mathlib.Analysis.Complex.Basic

Formal statement

theorem lean_workbook_plus_3121 (n : ℕ) : n * (n - 1) = 2 * choose n 2   :=  by sorry

Source