Lean-Workbook Plus 72417: Trigonometric identity

Proved

WorkbookRestored.plus_72417

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

lean-workbookrestored-importssource-checked

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

Source: Lean-Workbook row lean_workbook_plus_72417 (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_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/blob/main/lean_workbook.json, row lean_workbook_plus_72417; immutable original Prove2Me node 381c6b0e-8ef5-42e8-a89a-766cb0a31e75

View graph