Prove2Me

Lean-Workbook Plus 27452: Trigonometric identity

Proved
WorkbookRestored.plus_27452

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

lean-workbookrestored-importssource-checked

For every real xx, sin⁡(3x)=(1+2cos⁡(2x))sin⁡x\sin(3x)=(1+2\cos(2x))\sin x.

Source: Lean-Workbook row lean_workbook_plus_27452 (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_27452 (x : ℝ) : Real.sin (3*x) = (1 + 2*Real.cos (2*x))*Real.sin x   :=  by sorry
Source
https://huggingface.co/datasets/internlm/Lean-Workbook/blob/main/lean_workbook.json, row lean_workbook_plus_27452; immutable original Prove2Me node eec86142-595b-451f-8c59-d05295f91289

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