Let $n$ be a positive integer. Determine all positive real numbers $x$ satisfying
$nx^2 +\frac{2^2}{x + 1}+\frac{3^2}{x + 2}+...+\frac{(n + 1)^2}{x + n}= nx + \frac{n(n + 3)}{2}$
1. Start with the given equation:
   \[
   nx^2 + \frac{2^2}{x + 1} + \frac{3^2}{x + 2} + \ldots + \frac{(n + 1)^2}{x + n} = nx + \frac{n(n + 3)}{2}
   \]

2. Subtract the right-hand side (RHS) from the left-hand side (LHS) to set the equation to zero:
   \[
   nx^2 + \frac{2^2}{x + 1} + \frac{3^2}{x + 2} + \ldots + \frac{(n + 1)^2}{x + n} - \left(nx + \frac{n(n + 3)}{2}\right) = 0
   \]

3. Rearrange the terms:
   \[
   nx^2 - nx + \frac{2^2}{x + 1} + \frac{3^2}{x + 2} + \ldots + \frac{(n + 1)^2}{x + n} - \frac{n(n + 3)}{2} = 0
   \]

4. Factor out \(x - 1\) from the polynomial part:
   \[
   (x - 1)\left[nx - \left(\frac{2}{x + 1} + \frac{3}{x + 2} + \ldots + \frac{n + 1}{x + n}\right)\right] = 0
   \]

5. This gives us two cases to consider:
   - Case 1: \(x - 1 = 0\)
     \[
     x = 1
     \]
   - Case 2: 
     \[
     nx = \frac{2}{x + 1} + \frac{3}{x + 2} + \ldots + \frac{n + 1}{x + n}
     \]

6. For Case 2, assume \(x \neq 1\). We need to show that this leads to a contradiction:
   \[
   nx = \sum_{i=1}^n \frac{i + 1}{x + i}
   \]

7. Consider the sum on the right-hand side:
   \[
   \sum_{i=1}^n \frac{i + 1}{x + i}
   \]

8. Notice that each term \(\frac{i + 1}{x + i}\) is positive for \(x > 0\). Therefore, the sum of positive terms is positive:
   \[
   \sum_{i=1}^n \frac{i + 1}{x + i} > 0
   \]

9. Since \(nx\) is also positive for \(x > 0\), we need to check if the equality holds:
   \[
   nx = \sum_{i=1}^n \frac{i + 1}{x + i}
   \]

10. For \(x = 1\), we can verify:
    \[
    n \cdot 1 = \sum_{i=1}^n \frac{i + 1}{1 + i}
    \]
    \[
    n = \sum_{i=1}^n 1 = n
    \]

11. Therefore, \(x = 1\) satisfies the equation. For \(x \neq 1\), the sum \(\sum_{i=1}^n \frac{i + 1}{x + i}\) cannot equal \(nx\) because the terms are not balanced.

12. Hence, the only solution is \(x = 1\).

\(\blacksquare\)

The final answer is \( \boxed{ x = 1 } \)