Prove2Me

Lean-Workbook Plus 15898: Trigonometric inequality

Proved
WorkbookRestored.plus_15898

by wamlart · Sep 12, 2026 · Mathlib 777aaa6 (Lean v4.29.0-rc3)

lean-workbookrestored-importssource-checked

For every real α\alpha, −1≤cos⁡α≤1-1\le\cos\alpha\le1.

Source: Lean-Workbook row lean_workbook_plus_15898 (Apache-2.0). The unchanged proposition comes from the original record; its missing mathematical imports and namespaces are restored here.

Preamble
import Mathlib.Analysis.SpecialFunctions.Trigonometric.Basic
open Real
Formal statement
theorem WorkbookRestored.plus_15898 (α : ℝ) : -1 ≤ Real.cos α ∧ Real.cos α ≤ 1   :=  by sorry
Source
https://huggingface.co/datasets/internlm/Lean-Workbook/blob/main/lean_workbook.json, row lean_workbook_plus_15898; immutable original Prove2Me node 61e7434b-f3cd-4a04-8453-1dfd8d79737b

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