lean_workbook_plus_72417

Open

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

Prove that $\cos(2\pi/7)+\cos(4\pi/7)+\cos(6\pi/7) = -(cos(\frac{\pi}{7})+cos(\frac{3 \pi}{7})+cos(\frac{5 \pi}{7}))$

Preamble

import Mathlib.Analysis.Complex.Basic

Formal statement

theorem lean_workbook_plus_72417 :  Real.cos (2 * π / 7) + Real.cos (4 * π / 7) + Real.cos (6 * π / 7) = - (Real.cos (π / 7) + Real.cos (3 * π / 7) + Real.cos (5 * π / 7))   :=  by sorry

Source

https://huggingface.co/datasets/internlm/Lean-Workbook

View graph