Evaluate: `inttanxtan2xtan3x\ dx`

Explanation

To evaluate the integral \( \int \tan x \tan 2x \tan 3x \, dx \), we can use the tangent addition formula and properties of integrals. Here’s a step-by-step solution:

### Step 1: Rewrite \( \tan 3x \)
We can express \( \tan 3x \) using the tangent addition formula:
\[ \tan 3x = \tan(2x + x) = \frac{\tan 2x + \tan x}{1 - \tan 2x \tan x} \]

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