Evaluate `cosacos2acos3a......... cos 999 a ,` where `a=(2pi)/(1999)dot`

Explanation

To evaluate the expression \( \cos a \cos 2a \cos 3a \ldots \cos 999a \) where \( a = \frac{2\pi}{1999} \), we can follow these steps:

### Step 1: Define the Product
Let:
\[ P = \cos a \cos 2a \cos 3a \ldots \cos 999a \]

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