这道题怎么做辅助线解答？---**Question 17** **Total Score:** 15 points **Question Stem:** As shown in the figure, in the quadrangular pyramid P-ABCD, PA ⊥ plane ABCD, BC // AD, AB ⊥ AD. **(1) Proof:** Prove that plane PAB ⊥ plane PAD. **(2) Given Conditions:** If PA = AB = √2, AD = √3 + 1, BC = 2. Points P, B, C, D lie on the same sphere. Let the center of this sphere be O. **(i) Proof:** Prove that O is on plane ABCD. **(ii) Calculation:** Find the cosine value of the angle formed by line AC and line PO. --- **Chart/Diagram Description:** * **Type:** A three-dimensional geometric figure, specifically a quadrangular pyramid. * **Main Elements:** * **Vertices:** Labeled P (apex), A, B, C, D (base vertices). * **Edges/Lines:** * **Solid lines:** Represent visible edges: PB, PC, PD, AB, BC, CD. * **Dashed lines:** Represent hidden edges or auxiliary lines: PA (the height from P to A), AC (a diagonal in the base), AD (an edge of the base). * **Shapes:** * The overall figure is a pyramid P-ABCD. * The base ABCD is a quadrilateral. Given BC // AD and AB ⊥ AD, the base ABCD is a right trapezoid where AB is perpendicular to AD. * **Relative Position and Direction:** * Vertex P is positioned directly above vertex A, indicating PA is the height. * The base ABCD is below P, with A, B, C, D forming a planar figure.