Let $ABC$ be an acute-angled triangle and $P$ be a point in its interior. Let $P_A,P_B$ and $P_c$ be the images of $P$ under reflection in the sides $BC,CA$, and $AB$, respectively. If $P$ is the orthocentre of the triangle $P_AP_BP_C$ and if the largest angle of the triangle that can be formed by the line segments$ PA, PB$. and $PC$ is $x^o$, determine the value of $x$.
1. **Understanding the Problem:**
   We are given an acute-angled triangle \( \triangle ABC \) and a point \( P \) inside it. The points \( P_A, P_B, P_C \) are the reflections of \( P \) across the sides \( BC, CA, \) and \( AB \) respectively. We are also given that \( P \) is the orthocenter of \( \triangle P_A P_B P_C \). We need to determine the largest angle formed by the line segments \( PA, PB, \) and \( PC \).

2. **Using the Reflection Property:**
   By reflecting \( P \) across the sides of \( \triangle ABC \), we get:
   - \( P_A \) is the reflection of \( P \) across \( BC \).
   - \( P_B \) is the reflection of \( P \) across \( CA \).
   - \( P_C \) is the reflection of \( P \) across \( AB \).

3. **Orthocenter Property:**
   Given that \( P \) is the orthocenter of \( \triangle P_A P_B P_C \), we can use the property that the orthocenter of a triangle is the point where the altitudes intersect. This implies that the perpendiculars from \( P \) to \( BC, CA, \) and \( AB \) are the altitudes of \( \triangle P_A P_B P_C \).

4. **Circumcenter and Symmetry:**
   Since \( P \) is the orthocenter of \( \triangle P_A P_B P_C \), it implies that \( P \) is also the circumcenter of \( \triangle ABC \). This is because the reflections of the orthocenter across the sides of a triangle lie on the circumcircle of the triangle.

5. **Angle Calculation:**
   To find the largest angle formed by \( PA, PB, \) and \( PC \), we note that \( P \) being the circumcenter of \( \triangle ABC \) means that \( PA, PB, \) and \( PC \) are radii of the circumcircle of \( \triangle ABC \). The largest angle in a triangle formed by three radii of a circle is \( 120^\circ \).

6. **Conclusion:**
   Therefore, the largest angle formed by the line segments \( PA, PB, \) and \( PC \) is \( 120^\circ \).

The final answer is \( \boxed{120^\circ} \).