Determine all positive integers $n < 200$, such that $n^2 + (n+ 1)^2$ is the square of an integer.
To determine all positive integers \( n < 200 \) such that \( n^2 + (n+1)^2 \) is the square of an integer, we need to find \( n \) such that \( n^2 + (n+1)^2 = k^2 \) for some integer \( k \).

1. Start with the given equation:
   \[
   n^2 + (n+1)^2 = k^2
   \]

2. Expand and simplify the left-hand side:
   \[
   n^2 + (n+1)^2 = n^2 + n^2 + 2n + 1 = 2n^2 + 2n + 1
   \]
   Thus, we need:
   \[
   2n^2 + 2n + 1 = k^2
   \]

3. Rearrange the equation to form a quadratic in \( n \):
   \[
   2n^2 + 2n + 1 - k^2 = 0
   \]
   \[
   2n^2 + 2n + (1 - k^2) = 0
   \]

4. For \( n \) to be an integer, the discriminant of this quadratic equation must be a perfect square. The discriminant \( \Delta \) of the quadratic equation \( 2n^2 + 2n + (1 - k^2) = 0 \) is given by:
   \[
   \Delta = b^2 - 4ac = 2^2 - 4 \cdot 2 \cdot (1 - k^2) = 4 - 8(1 - k^2) = 4 + 8k^2 - 8 = 8k^2 - 4
   \]
   \[
   \Delta = 8k^2 - 4
   \]

5. For \( n \) to be an integer, \( \Delta \) must be a perfect square. Let \( \Delta = m^2 \) for some integer \( m \):
   \[
   8k^2 - 4 = m^2
   \]
   \[
   m^2 = 8k^2 - 4
   \]

6. Rearrange to solve for \( k \):
   \[
   m^2 + 4 = 8k^2
   \]
   \[
   m^2 + 4 \equiv 0 \pmod{8}
   \]

7. Check for values of \( m \) such that \( m^2 + 4 \) is divisible by 8. We find that \( m \) must be even. Let \( m = 2p \):
   \[
   (2p)^2 + 4 = 8k^2
   \]
   \[
   4p^2 + 4 = 8k^2
   \]
   \[
   p^2 + 1 = 2k^2
   \]

8. Solve the equation \( p^2 + 1 = 2k^2 \) for integer values of \( p \) and \( k \). This is a Pell's equation. The fundamental solution to this equation is \( (p, k) = (1, 1) \). Using the theory of Pell's equations, we can generate further solutions.

9. The solutions to the Pell's equation \( p^2 + 1 = 2k^2 \) are given by:
   \[
   p_n + k_n\sqrt{2} = (1 + \sqrt{2})^n
   \]

10. Calculate the first few solutions:
    - For \( n = 1 \): \( (p, k) = (1, 1) \)
    - For \( n = 2 \): \( (p, k) = (7, 5) \)
    - For \( n = 3 \): \( (p, k) = (41, 29) \)
    - For \( n = 4 \): \( (p, k) = (239, 169) \)

11. Check the corresponding \( n \) values:
    - For \( p = 1 \), \( n = 3 \)
    - For \( p = 7 \), \( n = 20 \)
    - For \( p = 41 \), \( n = 119 \)

12. Verify that these values of \( n \) are less than 200 and satisfy the original equation.

The final answer is \( \boxed{ n = 3, 20, 119 } \).