Prove Theorem 3.1 in the case where the line through P perpendicular to BC is tangent to the circumcircle of?

Question

triangle ABC. In this case, P and Q coincide, and you must show that PA is parallel to XZ.

Theorem 3.1 : Choose a point P on the circumcircle of triangle ABC and let Q be the other point where the perpendicular to BC through P meets the circumcircle. Let X be the point where this perpendicular meets line BC and let Z be the point where the perpendicular to AB through P meets AB. If Q is different from A, then Z lies on the line parallel to QA through X.

Answer 1

I don't see how the perpendicular to BC can be tangent to the circumcircle.