Let $ n$ and $ k$ be given natural numbers, and let $ A$ be a set such that \[ |A| \leq \frac{n(n+1)}{k+1}.\] For $ i=1,2,...,n+1$, let $ A_i$ be sets of size $ n$ such that \[ |A_i \cap A_j| \leq k \;(i \not=j)\ ,\] \[ A=  \bigcup_{i=1}^{n+1} A_i.\] Determine the cardinality of $ A$. 

[i]K. Corradi[/i]
1. **Assume the set \( A = \{1, 2, \dots, m\} \).**
   - Let \( b_i \) (for \( i = 1, 2, \dots, m \)) denote the number of occurrences of element \( i \) among all the sets \( A_j \) (for \( j = 1, 2, \dots, n+1 \)).

2. **Calculate the sum of intersections:**
   - We know that \( |A_i \cap A_j| \leq k \) for \( i \neq j \).
   - The total number of pairs \((i, j)\) where \( i \neq j \) is \( \binom{n+1}{2} = \frac{(n+1)n}{2} \).
   - Therefore, \( k \binom{n+1}{2} \geq \sum_{i > j} |A_i \cap A_j| \).

3. **Express the sum of intersections in terms of \( b_i \):**
   - The sum of intersections can also be written as \( \sum_{i=1}^m \binom{b_i}{2} \).
   - Hence, \( k \frac{n(n+1)}{2} \geq \sum_{i=1}^m \binom{b_i}{2} \).

4. **Simplify the binomial coefficient:**
   - Recall that \( \binom{b_i}{2} = \frac{b_i(b_i - 1)}{2} \).
   - Therefore, \( k \frac{n(n+1)}{2} \geq \sum_{i=1}^m \frac{b_i(b_i - 1)}{2} \).

5. **Use the inequality involving the sum of squares:**
   - We can use the inequality \( \sum_{i=1}^m \binom{b_i}{2} \geq \frac{1}{m} \left( \sum_{i=1}^m b_i \right)^2 - \frac{m}{4} \).
   - This follows from the Cauchy-Schwarz inequality and the fact that \( \sum_{i=1}^m b_i = n(n+1) \).

6. **Substitute the sum of \( b_i \):**
   - We have \( \sum_{i=1}^m b_i = n(n+1) \).
   - Therefore, \( k \frac{n(n+1)}{2} \geq \frac{1}{m} \left( n(n+1) \right)^2 - \frac{m}{4} \).

7. **Simplify the inequality:**
   - Multiply both sides by 2 to clear the fraction: \( kn(n+1) \geq \frac{1}{m} \left( n(n+1) \right)^2 - \frac{m}{2} \).
   - Rearrange the terms: \( kn(n+1) \geq \frac{n^2(n+1)^2}{m} - \frac{m}{2} \).

8. **Multiply through by \( m \):**
   - \( kmn(n+1) \geq n^2(n+1)^2 - \frac{m^2}{2} \).

9. **Rearrange to isolate \( m \):**
   - \( kmn(n+1) + \frac{m^2}{2} \geq n^2(n+1)^2 \).
   - Multiply through by 2: \( 2kmn(n+1) + m^2 \geq 2n^2(n+1)^2 \).

10. **Solve the quadratic inequality:**
    - \( m^2 + 2kmn(n+1) \geq 2n^2(n+1)^2 \).
    - This implies \( m \geq \frac{n(n+1)}{k+1} \).

11. **Conclusion:**
    - Since \( |A| \leq \frac{n(n+1)}{k+1} \) and we have shown \( m \geq \frac{n(n+1)}{k+1} \), it follows that \( |A| = \frac{n(n+1)}{k+1} \).

The final answer is \( \boxed{\frac{n(n+1)}{k+1}} \).