1. Understanding the Problem:
We are given two positive integers $a$ and $b$ with $\gcd(a, b) = 1$. We need to show that every integer greater than $ab - a - b$ can be expressed in the form $ax + by$, where $x, y \in \mathbb{N}_0$. This is a classic result in number theory known as the Frobenius Coin Problem for two variables.

2. Frobenius Number for Two Variables:
The Frobenius number $g(a, b)$ for two coprime integers $a$ and $b$ is given by:
$$g(a, b) = ab - a - b$$
The theorem states that any integer greater than $g(a, b)$ can be expressed as $ax + by$ for non-negative integers $x$ and $y$.

3. Proof for Two Variables:
- Since $\gcd(a, b) = 1$, by the Extended Euclidean Algorithm, there exist integers $x_0$ and $y_0$ such that:
$$ax_0 + by_0 = 1$$
- For any integer $n > ab - a - b$, we can write $n$ as:
$$n = ab - a - b + k \quad \text{for some integer } k > 0$$
- We need to show that $n$ can be written in the form $ax + by$ with $x, y \in \mathbb{N}_0$. Consider:
$$n = a(b-1) + b(a-1) + k$$
- We can rewrite $n$ as:
$$n = a(b-1 + k_1) + b(a-1 + k_2)$$
where $k_1$ and $k_2$ are non-negative integers such that $k_1 + k_2 = k$. This shows that $n$ can be expressed as $ax + by$ for non-negative integers $x$ and $y$.

4. Extension to Three Variables:
- For three pairwise relatively prime positive integers $a, b,$ and $c$, we need to show that $ax + by + cz = n$ has non-negative integer solutions $(x, y, z)$ for integers $n > abc - ab - bc - ca + a + b + c$.
- The Frobenius number for three variables is not as straightforward as for two variables. However, a similar result can be conjectured:
$$g(a, b, c) = abc - ab - bc - ca + a + b + c$$
- The proof for three variables involves more complex combinatorial arguments and is beyond the scope of elementary number theory. However, it is known that for three pairwise relatively prime integers, there exists a bound beyond which every integer can be expressed as a non-negative linear combination of $a, b,$ and $c$.

5. **Generalization to $k$ Variables:**
- For $k$ pairwise relatively prime positive integers, the problem becomes even more complex. The Frobenius number for $k$ variables does not have a simple closed-form expression.
- However, it is known that for any set of $k$ pairwise relatively prime integers, there exists a bound beyond which every integer can be expressed as a non-negative linear combination of these integers.

$\blacksquare$