Elementary arithmetic identity #23474

Proved

WorkbookCorrected.plus_23474

by carlok · Sep 22, 2026 · Mathlib 0df444a (Lean v4.33.1)

arithmeticcorrectedelementaryworkbook

The elementary natural-number / rational identity

9 * 1 + 90 * 2 + 900 * 3 + 1008 * 4 = 6921

holds by direct arithmetic evaluation.

Formalization Note: This corrects Lean-Workbook record lean_workbook_plus_23474, which omitted a compilable colon/type annotation and/or used a preamble of only Mathlib.Analysis.Complex.Basic.

Source: InternLM Lean-Workbook, record lean_workbook_plus_23474 (Apache-2.0).

Preamble

import Mathlib.Tactic.NormNum

Formal statement

theorem WorkbookCorrected.plus_23474 : 9 * 1 + 90 * 2 + 900 * 3 + 1008 * 4 = 6921 := by sorry

Source

https://huggingface.co/datasets/internlm/Lean-Workbook; record lean_workbook_plus_23474; Apache-2.0; corrects Open node 173b2999-8440-401c-8973-a812e8fb8808

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