Given a regular polygon with $n$ sides. It is known that there are $1200$ ways to choose three of the vertices of the polygon such that they form the vertices of a [b]right triangle[/b]. What is the value of $n$?
1. **Understanding the problem**: We need to find the number of sides \( n \) of a regular polygon such that there are 1200 ways to choose three vertices that form a right triangle.

2. **Key observation**: In a regular polygon, a right triangle can be formed if and only if one of its sides is the diameter of the circumscribed circle. This implies that \( n \) must be even, because only then can we have diametrically opposite vertices.

3. **Counting the hypotenuses**: Since the polygon is regular and has \( n \) sides, there are \( \frac{n}{2} \) possible diameters (each pair of diametrically opposite vertices forms a diameter).

4. **Counting the right triangles**: For each diameter, the remaining \( n-2 \) vertices can be paired with the endpoints of the diameter to form right triangles. Therefore, each diameter generates \( n-2 \) right triangles.

5. **Setting up the equation**: The total number of right triangles is given by the product of the number of diameters and the number of right triangles per diameter:
   \[
   \frac{n}{2} \times (n-2) = 1200
   \]

6. **Solving the equation**:
   \[
   \frac{n}{2} \times (n-2) = 1200
   \]
   \[
   \frac{n(n-2)}{2} = 1200
   \]
   \[
   n(n-2) = 2400
   \]
   \[
   n^2 - 2n - 2400 = 0
   \]

7. **Solving the quadratic equation**: We solve the quadratic equation \( n^2 - 2n - 2400 = 0 \) using the quadratic formula \( n = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a} \), where \( a = 1 \), \( b = -2 \), and \( c = -2400 \):
   \[
   n = \frac{-(-2) \pm \sqrt{(-2)^2 - 4 \cdot 1 \cdot (-2400)}}{2 \cdot 1}
   \]
   \[
   n = \frac{2 \pm \sqrt{4 + 9600}}{2}
   \]
   \[
   n = \frac{2 \pm \sqrt{9604}}{2}
   \]
   \[
   n = \frac{2 \pm 98}{2}
   \]

8. **Finding the roots**:
   \[
   n = \frac{2 + 98}{2} = 50
   \]
   \[
   n = \frac{2 - 98}{2} = -48
   \]

9. **Selecting the valid solution**: Since \( n \) must be a positive integer, we discard \( n = -48 \) and accept \( n = 50 \).

The final answer is \( \boxed{50} \).