讲解一下---**Extraction Content:** **Question 17:** (15分) As shown in the figure, in the quadrangular pyramid P-ABCD, PA ⊥ plane ABCD, BC // AD, AB ⊥ AD. (1) Prove: plane PAB ⊥ plane PAD; (2) If PA = AB = $\sqrt{2}$, AD = $\sqrt{3}+1$, BC = 2, points P, B, C, D are on the same sphere, let the center of the sphere be O. (i) Prove: O is on plane ABCD; (ii) Find the cosine value of the angle between line AC and line PO. **Chart/Diagram Description:** * **Type:** A 3D geometric figure, specifically a pyramid. * **Main Elements:** * **Vertices:** Labeled points P, A, B, C, D. P is the apex. A, B, C, D are points in the base plane. * **Edges:** * Solid lines represent visible edges: PB, PC, PD, AB, BC, CD, AD. * Dashed line represents a hidden edge: PA. * **Base:** The base is the quadrilateral ABCD. It is drawn in perspective, appearing as a non-rectangular shape. The relative positions suggest that A is in the foreground-left, B in the foreground-right, C in the background-right, and D in the background-left, as viewed from the front. * **Apex:** P is positioned above point A, connected by the dashed line PA, indicating PA is the height and perpendicular to the base. * **Labels:** Vertices are labeled with capital letters P, A, B, C, D.