Prove that:
`cotAcot2A-cot2Acot3A-cot3AcotA=1`

Explanation

To prove the identity \( \cot A \cot 2A - \cot 2A \cot 3A - \cot 3A \cot A = 1 \), we will use the cotangent addition formula and properties of cotangent.

### Step 1: Rewrite \( \cot 3A \)
Using the cotangent addition formula, we can express \( \cot 3A \) as:
\[ \cot 3A = \cot(2A + A) = \frac{\cot 2A \cot A - 1}{\cot 2A + \cot A} \]

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