9. Assume that the parallel lines I and l' have a common perpendicu- lar PỎ. For any point X on 1, let X' be the foot of the perpendicu- lar from X to l'. Prove that as X recedes endlessly from P on 1, the segment XX' increases indefinitely; see Figure 6.20. (Hint: We saw that it increases in Exercise 4. Drop a perpendicular XY to the lim- iting parallel ray between PX and PX'. Use the crossbar theorem to show that PỶ intersects XX' in a point Z. Use Proposition 4.5 to show that XZ XY. Conclude by applying Aristotle's axiom.) Figure 6.20 P X Y Z } Q X'