Prove2Me

Lean-Workbook Plus 28174: Trigonometric identity

Proved
WorkbookRestored.plus_28174

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

lean-workbookrestored-importssource-checked

If α=π−(β+γ)\alpha=\pi-(\beta+\gamma), then cos⁡α=−cos⁡(γ+β)\cos\alpha=-\cos(\gamma+\beta).

Source: Lean-Workbook row lean_workbook_plus_28174 (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_28174 (α β γ : ℝ) : α = π - (β + γ) → cos α = -cos (γ + β)   :=  by sorry
Source
https://huggingface.co/datasets/internlm/Lean-Workbook/blob/main/lean_workbook.json, row lean_workbook_plus_28174; immutable original Prove2Me node dd016d64-f144-4085-a2f2-e67d7b3c3838

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