Let $S=\{a+np : n=0,1,2,3,... \}$ where $a$ is a positive integer and $p$ is a prime. Suppose there exist positive integers $x$ and $y$ st $x^{41}$ and $y^{49}$ are in $S$. Determine if there exists a positive integer $z$ st $z^{2009}$ is in $S$.
1. **Given Set Definition**: 
   We start with the set \( S = \{a + np : n = 0, 1, 2, 3, \ldots \} \), where \( a \) is a positive integer and \( p \) is a prime number.

2. **Conditions on \( x \) and \( y \)**:
   We are given that there exist positive integers \( x \) and \( y \) such that \( x^{41} \) and \( y^{49} \) are in \( S \). This implies:
   \[
   x^{41} \equiv a \pmod{p} \quad \text{and} \quad y^{49} \equiv a \pmod{p}
   \]

3. **Objective**:
   We need to determine if there exists a positive integer \( z \) such that \( z^{2009} \) is in \( S \). This translates to finding \( z \) such that:
   \[
   z^{2009} \equiv a \pmod{p}
   \]

4. **Case Analysis**:
   - **Case 1: \( p \mid a \)**:
     If \( p \mid a \), then \( a \equiv 0 \pmod{p} \). We can choose \( z \equiv 0 \pmod{p} \). Thus, \( z^{2009} \equiv 0 \equiv a \pmod{p} \).

   - **Case 2: \( p \nmid a \)**:
     If \( p \nmid a \), we need to find \( z \) such that \( z^{2009} \equiv a \pmod{p} \). We can express \( z \) in terms of \( x \) and \( y \):
     \[
     z \equiv x^b y^c \pmod{p}
     \]
     for some integers \( b \) and \( c \). Then:
     \[
     z^{2009} \equiv (x^b y^c)^{2009} \equiv x^{2009b} y^{2009c} \equiv a^{49b} a^{41c} \equiv a^{49b + 41c} \pmod{p}
     \]
     We need \( 49b + 41c \equiv 1 \pmod{\varphi(p)} \), where \( \varphi(p) = p-1 \) since \( p \) is prime. Since \(\gcd(49, 41) = 1\), by the Extended Euclidean Algorithm, there exist integers \( b \) and \( c \) such that:
     \[
     49b + 41c = 1
     \]
     Therefore, we can find such \( b \) and \( c \) to satisfy the congruence.

5. **Ensuring \( z^{2009} \geq a \)**:
   We need \( z \) such that \( z^{2009} \geq a \). Assume \( z \equiv d \pmod{p} \) where \( d \) is the residue class modulo \( p \). We can choose \( z \) such that:
   \[
   z \equiv d \pmod{p} \quad \text{and} \quad z \geq a
   \]
   This ensures \( z^{2009} \geq a^{2009} \geq a \).

Conclusion:
Since we can always find such \( z \) that satisfies the conditions, there exists a positive integer \( z \) such that \( z^{2009} \) is in \( S \).

The final answer is \( \boxed{\text{True}} \)