lean_workbook_plus_12106

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

Prove that $\sin(a)\cos(a)+\sin(b)\cos(b)=\sin(a+b)\cos(a-b)$

Preamble

import Mathlib.Analysis.Complex.Basic

Formal statement

theorem lean_workbook_plus_12106 (a b : ℝ) : sin a * cos a + sin b * cos b = sin (a + b) * cos (a - b)   :=  by sorry

Source