Let $p, q, r$ be prime numbers and $t, n$ be natural numbers such that $p^2 +qt =(p + t)^n$ and $p^2 + qr = t^4$ .
a) Show that $n < 3$.
b) Determine all the numbers $p, q, r, t, n$ that satisfy the given conditions.
Given the equations:
\[ p^2 + qt = (p + t)^n \]
\[ p^2 + qr = t^4 \]

We need to show that \( n < 3 \) and determine all the numbers \( p, q, r, t, n \) that satisfy the given conditions.

### Part (a): Show that \( n < 3 \)

1. Assume \( n \geq 3 \). Then:
   \[ (p + t)^n \geq (p + t)^3 \]

2. Expanding \( (p + t)^3 \) using the binomial theorem:
   \[ (p + t)^3 = p^3 + 3p^2t + 3pt^2 + t^3 \]

3. Since \( p^2 + qt = (p + t)^n \), we have:
   \[ p^2 + qt \leq (p + t)^n \]

4. For \( n \geq 3 \):
   \[ (p + t)^n \geq p^3 + 3p^2t + 3pt^2 + t^3 \]

5. Comparing the terms:
   \[ p^2 + qt \leq p^3 + 3p^2t + 3pt^2 + t^3 \]

6. Since \( qt \leq 3p^2t + 3pt^2 + t^3 \), and \( qt \) is linear in \( t \) while the right-hand side is cubic in \( t \), it is clear that:
   \[ p^2 + qt < p^3 + 3p^2t + 3pt^2 + t^3 \]

7. This leads to a contradiction because \( p^2 + qt \) cannot be greater than or equal to \( (p + t)^n \) for \( n \geq 3 \).

8. Therefore, \( n < 3 \).

### Part (b): Determine all the numbers \( p, q, r, t, n \)

1. From part (a), we know \( n \leq 2 \). Since \( p^2 + qt > p + t \), \( n \) cannot be 1. Thus, \( n = 2 \).

2. Given \( n = 2 \):
   \[ p^2 + qt = (p + t)^2 \]
   \[ p^2 + qt = p^2 + 2pt + t^2 \]

3. Simplifying:
   \[ qt = 2pt + t^2 \]
   \[ t(q - 2p) = t^2 \]
   \[ t(q - 2p - t) = 0 \]

4. Since \( t \neq 0 \):
   \[ q - 2p = t \]

5. Substituting \( t = q - 2p \) into the second equation:
   \[ p^2 + qr = (q - 2p)^4 \]

6. Simplifying modulo \( q \):
   \[ p^2 + qr \equiv 0 \pmod{q} \]
   \[ (q - 2p)^4 \equiv 0 \pmod{q} \]

7. Since \( q \) is a prime number:
   \[ q \mid (q - 2p)^4 \]
   \[ q \mid 16p^4 - p^2 \]
   \[ q \mid p^2(4p - 1)(4p + 1) \]

8. Since \( q > 2p \):
   \[ q \mid (4p - 1)(4p + 1) \]

9. Considering \( q \geq 4p - 1 \) and \( q \geq 4p + 1 \):
   \[ q = 4p - 1 \text{ or } q = 4p + 1 \]

10. For \( q = 4p - 1 \):
    \[ p^2 + (4p - 1)r = (2p - 1)^4 \]
    \[ r = \frac{(2p - 1)^4 - p^2}{4p - 1} \]

11. For \( p = 2 \):
    \[ q = 4(2) - 1 = 7 \]
    \[ t = q - 2p = 7 - 4 = 3 \]
    \[ r = \frac{(2(2) - 1)^4 - 2^2}{4(2) - 1} = \frac{3^4 - 4}{7} = \frac{81 - 4}{7} = 11 \]

12. Thus, \( (p, q, r, t, n) = (2, 7, 11, 3, 2) \).

13. For \( q = 4p + 1 \):
    \[ r = \frac{(2p + 1)^4 - p^2}{4p + 1} \]
    \[ r = (p + 1)(4p^2 + 3p + 1) \]

14. Since \( r \) must be prime, \( 4p^2 + 3p + 1 \) must be 1, which is not possible for any prime \( p \).

The final answer is \( \boxed{ (p, q, r, t, n) = (2, 7, 11, 3, 2) } \)