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Lean-Workbook Plus 19213: Trigonometric identity

Proved
WorkbookRestored.plus_19213

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

lean-workbookrestored-importssource-checked

For every real xx, sin⁡(3x)=4sin⁡xsin⁡(π/3−x)sin⁡(π/3+x)\sin(3x)=4\sin x\sin(\pi/3-x)\sin(\pi/3+x).

Source: Lean-Workbook row lean_workbook_plus_19213 (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_19213 (x : ℝ) : Real.sin (3*x) = 4*Real.sin x * Real.sin (π/3 - x) * Real.sin (π/3 + x)   :=  by sorry
Source
https://huggingface.co/datasets/internlm/Lean-Workbook/blob/main/lean_workbook.json, row lean_workbook_plus_19213; immutable original Prove2Me node 6fc415df-d9f1-4303-be17-abe3a2b6e349

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