lean_workbook_plus_5165

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

Prove that $\tan^2\frac{\theta}{2}=\frac{1-\cos\theta}{1+\cos\theta}$ .

Preamble

import Mathlib.Analysis.Complex.Basic

Formal statement

theorem lean_workbook_plus_5165 : ∀ θ : ℝ, tan (θ / 2) ^ 2 = (1 - cos θ) / (1 + cos θ)   :=  by sorry

Source