Find all couples of polynomials $(P,Q)$ with real coefficients, such that for infinitely many $x\in\mathbb R$ the condition \[ \frac{P(x)}{Q(x)}-\frac{P(x+1)}{Q(x+1)}=\frac{1}{x(x+2)}\]
Holds.
[i] Nikolai Nikolov, Oleg Mushkarov[/i]
1. Given the equation:
   \[
   \frac{P(x)}{Q(x)} - \frac{P(x+1)}{Q(x+1)} = \frac{1}{x(x+2)}
   \]
   we start by clearing the denominators. Multiply both sides by \(Q(x)Q(x+1)x(x+2)\):
   \[
   x(x+2) \left( P(x)Q(x+1) - P(x+1)Q(x) \right) = Q(x)Q(x+1)
   \]
   Since this holds for infinitely many \(x \in \mathbb{R}\), it must hold for all \(x \in \mathbb{R}\) because \(P\) and \(Q\) are polynomials.

2. Define \(F(x) = \frac{P(x)}{Q(x)}\). Then the given equation becomes:
   \[
   F(x) - F(x+1) = \frac{1}{x(x+2)}
   \]

3. To find a general form for \(F(x)\), consider the recurrence relation:
   \[
   F(x+1) = F(x) - \frac{1}{x(x+2)}
   \]

4. Iterating this relation, we get:
   \[
   F(x+n+1) = F(x+n) - \frac{1}{(x+n)(x+n+2)}
   \]
   Summing these from \(n = 0\) to \(n = k-1\), we obtain:
   \[
   F(x+k) = F(x) - \sum_{n=0}^{k-1} \frac{1}{(x+n)(x+n+2)}
   \]

5. As \(k \to \infty\), the sum \(\sum_{n=0}^{k-1} \frac{1}{(x+n)(x+n+2)}\) converges to a constant because the terms \(\frac{1}{(x+n)(x+n+2)}\) decrease rapidly. Let this constant be \(C\). Thus:
   \[
   F(x) - \frac{1}{2} \left( \frac{1}{x} + \frac{1}{x+1} \right) = C
   \]

6. Solving for \(F(x)\), we get:
   \[
   F(x) = C + \frac{1}{2} \left( \frac{1}{x} + \frac{1}{x+1} \right)
   \]

7. Simplifying \(F(x)\):
   \[
   F(x) = C + \frac{1}{2} \left( \frac{x+1 + x}{x(x+1)} \right) = C + \frac{1}{2} \left( \frac{2x+1}{x(x+1)} \right)
   \]
   \[
   F(x) = C + \frac{1}{x(x+1)}
   \]

8. Therefore, the general form of \(F(x)\) is:
   \[
   F(x) = \frac{P(x)}{Q(x)} = C + \frac{1}{x(x+1)}
   \]

9. To find \(P(x)\) and \(Q(x)\), we can write:
   \[
   \frac{P(x)}{Q(x)} = \frac{2Cx^2 + 2(C+1)x + 1}{2x(x+1)}
   \]
   This implies:
   \[
   P(x) = 2Cx^2 + 2(C+1)x + 1 \quad \text{and} \quad Q(x) = 2x(x+1)
   \]

The final answer is \( \boxed{ (P(x), Q(x)) = (2Cx^2 + 2(C+1)x + 1, 2x(x+1)) } \).