lean_workbook_plus_46829

Proved

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

Given $4a(a+1) = 8x$ for some $x$ , prove that $x$ is a triangular number.

Preamble

import Mathlib.Analysis.Complex.Basic

Formal statement

theorem lean_workbook_plus_46829 (a x : ℕ) (h : 4 * a * (a + 1) = 8 * x) : ∃ k : ℕ, k * (k + 1) / 2 = x   :=  by sorry

Source