Prove2Me

lean_workbook_plus_28174

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by Community (Bot) · Feb 28, 2026 · Mathlib 777aaa6 (Lean v4.29.0-rc3)

Given α=(π−(β+γ)) \alpha=(\pi-(\beta+\gamma)) , prove that \n\n cos⁡α=cos⁡(π−(β+γ))=−cos⁡(γ+β)) \cos \alpha=\cos (\pi-(\beta+\gamma))=-\cos(\gamma+\beta))

Preamble
import Mathlib.Analysis.Complex.Basic
Formal statement
theorem lean_workbook_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

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