Evaluate: `intcos2xcos4xcos6x\ dx`

Explanation

To evaluate the integral \( \int \cos 2x \cos 4x \cos 6x \, dx \), we will use trigonometric identities to simplify the expression step by step.

### Step 1: Use the product-to-sum identities
We start by using the product-to-sum identity for cosine:
\[ \cos A \cos B = \frac{1}{2} \left( \cos(A+B) + \cos(A-B) \right) \]
We will first group \( \cos 2x \) and \( \cos 4x \).
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