Evaluate: `intcosxcos2xcos3x\ dx`

Explanation

To evaluate the integral \( I = \int \cos x \cos 2x \cos 3x \, dx \), we can use trigonometric identities to simplify the expression. Here’s a step-by-step solution:

### Step 1: Use the product-to-sum identities
We start by using the product-to-sum identity for the first two cosine terms:

\[ \cos A \cos B = \frac{1}{2} \left( \cos(A+B) + \cos(A-B) \right) \]
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