Let $ABC$ be a triangle . Let point $X$ be in the triangle and $AX$ intersects $BC$ in $Y$ . Draw the perpendiculars $YP,YQ,YR,YS$ to lines $CA,CX,BX,BA$ respectively. Find the necessary and sufficient condition for $X$ such that $PQRS$ be cyclic .
1. **Given**: We have a triangle \(ABC\) with a point \(X\) inside the triangle. The line segment \(AX\) intersects \(BC\) at point \(Y\). We draw perpendiculars from \(Y\) to the lines \(CA\), \(CX\), \(BX\), and \(BA\), and label the feet of these perpendiculars as \(P\), \(Q\), \(R\), and \(S\) respectively.

2. **Objective**: We need to find the necessary and sufficient condition for \(X\) such that the quadrilateral \(PQRS\) is cyclic.

3. **Cyclic Quadrilateral Condition**: For \(PQRS\) to be cyclic, the sum of opposite angles must be \(180^\circ\). Specifically, we need:
   \[
   \angle QPS + \angle SRQ = 180^\circ
   \]

4. **Angle Chasing**:
   - Since \(YP \perp CA\), \(YQ \perp CX\), \(YR \perp BX\), and \(YS \perp BA\), we have:
     \[
     \angle QPS = \angle QCY - \angle SPY
     \]
   - Since \(ASYP\) is cyclic, \(\angle SPY = \angle SAY = \angle BAY\).

5. **Using Cyclic Properties**:
   - Since \(YRSQ\) is cyclic, \(\angle YRQ = \angle YXQ\).
   - Therefore, we have:
     \[
     \angle QPS + \angle YRQ = \angle QCY - \angle BAY + \angle YXQ
     \]

6. **Simplifying the Angles**:
   - Note that \(\angle QCY = \angle YXC + \angle XCY\).
   - Thus:
     \[
     \angle QPS + \angle YRQ = \angle YXC + \angle XCY - \angle BAY + \angle YXQ
     \]
   - Since \(\angle YXQ = \angle YXC\), we get:
     \[
     \angle QPS + \angle YRQ = \angle YXC + \angle XCY - \angle BAY + \angle YXC
     \]
   - Simplifying further:
     \[
     \angle QPS + \angle YRQ = 2\angle YXC + \angle XCY - \angle BAY
     \]

7. **Condition for Cyclic Quadrilateral**:
   - For \(PQRS\) to be cyclic, we need:
     \[
     2\angle YXC + \angle XCY - \angle BAY = \angle B
     \]
   - Since \(\angle BYA = 90^\circ\), we have:
     \[
     \angle BYA = 90^\circ
     \]

8. **Conclusion**:
   - Therefore, the necessary and sufficient condition for \(X\) such that \(PQRS\) is cyclic is that \(X\) must lie on the altitude from \(A\) to \(BC\).

\(\blacksquare\)

The final answer is \( \boxed{ X } \) must lie on the altitude from \(A\) to \(BC\).