Show that in the plane there exists a convex polygon of 1992 sides satisfying the following conditions:

[i](i)[/i] its side lengths are $ 1, 2, 3, \ldots, 1992$ in some order;
[i](ii)[/i] the polygon is circumscribable about a circle.

[i]Alternative formulation:[/i] Does there exist a 1992-gon with side lengths $ 1, 2, 3, \ldots, 1992$ circumscribed about a circle? Answer the same question for a 1990-gon.
1. **Condition for Circumscribability:**
   A polygon is circumscribable about a circle if and only if the sum of the lengths of its opposite sides is equal. This is known as Pitot's theorem for polygons.

2. **Sum of Side Lengths:**
   The sum of the side lengths of the polygon is:
   \[
   S = 1 + 2 + 3 + \cdots + 1992 = \frac{1992 \times 1993}{2} = 1987026
   \]

3. **Checking for 1992-gon:**
   - Since 1992 is even, we can pair the sides such that the sum of each pair is equal. For example, we can pair the smallest and largest sides, the second smallest and the second largest, and so on.
   - Pairing the sides: \((1, 1992), (2, 1991), (3, 1990), \ldots, (996, 997)\).
   - Each pair sums to:
     \[
     1 + 1992 = 1993, \quad 2 + 1991 = 1993, \quad \ldots, \quad 996 + 997 = 1993
     \]
   - There are 996 pairs, and each pair sums to 1993. Thus, the sum of the lengths of opposite sides is equal, satisfying Pitot's theorem.

4. **Conclusion for 1992-gon:**
   Since we can pair the sides such that the sum of the lengths of opposite sides is equal, a 1992-gon with side lengths \(1, 2, 3, \ldots, 1992\) can be circumscribed about a circle.

5. **Checking for 1990-gon:**
   - For a 1990-gon, we need to check if we can pair the sides such that the sum of the lengths of opposite sides is equal.
   - The sum of the side lengths of the polygon is:
     \[
     S = 1 + 2 + 3 + \cdots + 1990 = \frac{1990 \times 1991}{2} = 1989055
     \]
   - Since 1990 is even, we can pair the sides similarly to the 1992-gon.
   - Pairing the sides: \((1, 1990), (2, 1989), (3, 1988), \ldots, (995, 996)\).
   - Each pair sums to:
     \[
     1 + 1990 = 1991, \quad 2 + 1989 = 1991, \quad \ldots, \quad 995 + 996 = 1991
     \]
   - There are 995 pairs, and each pair sums to 1991. Thus, the sum of the lengths of opposite sides is equal, satisfying Pitot's theorem.

6. **Conclusion for 1990-gon:**
   Since we can pair the sides such that the sum of the lengths of opposite sides is equal, a 1990-gon with side lengths \(1, 2, 3, \ldots, 1990\) can also be circumscribed about a circle.

The final answer is both a 1992-gon and a 1990-gon with side lengths \( \boxed{ 1, 2, 3, \ldots, 1992 } \) and \(1, 2, 3, \ldots, 1990\) respectively can be circumscribed about a circle.