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Lean-Workbook Plus 5722: Trigonometric identity

Proved
WorkbookRestored.plus_5722

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

lean-workbookrestored-importssource-checked

For all real a,ba,b, 2cos⁡acos⁡b=cos⁡(a+b)+cos⁡(a−b)2\cos a\cos b=\cos(a+b)+\cos(a-b).

Source: Lean-Workbook row lean_workbook_plus_5722 (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_5722 (a b : ℝ) : 2 * cos a * cos b = cos (a + b) + cos (a - b)   :=  by sorry
Source
https://huggingface.co/datasets/internlm/Lean-Workbook/blob/main/lean_workbook.json, row lean_workbook_plus_5722; immutable original Prove2Me node a00d7137-fd6b-4df6-8f10-ca24bebe6d2d

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