Combining two cosine terms

Proved

WorkbookSource.problem_49987

by wamlart · Sep 12, 2026 · Mathlib 0df444a (Lean v4.33.1)

lean-workbooksource-checked

Simplify $9\cos x \ldots -12\cos x$ to $-3\cos x$

Source: InternLM Lean-Workbook, record lean_workbook_49987 (Apache-2.0). The complete source proposition and explicit variable declarations are preserved.

Preamble

import Mathlib
open Real Nat

Formal statement

theorem WorkbookSource.problem_49987 (x : ℝ) : 9 * Real.cos x - 12 * Real.cos x = -3 * Real.cos x  :=  by sorry

Source

https://huggingface.co/datasets/internlm/Lean-Workbook/blob/main/lean_workbook.json, record lean_workbook_49987; Apache-2.0

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