Prove that: `tan^(-1)1+tan^(-1)2+tan^(-1)3=pi`

Explanation

To prove that \( \tan^{-1}(1) + \tan^{-1}(2) + \tan^{-1}(3) = \pi \), we will use the formula for the sum of inverse tangents:

\[ \tan^{-1}(a) + \tan^{-1}(b) = \tan^{-1}\left(\frac{a+b}{1-ab}\right) \quad \text{if } ab < 1 \]

### Step 1: Combine the first two terms
Let's first combine \( \tan^{-1}(1) \) and \( \tan^{-1}(2) \):
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