请解决这道题目---**Extracted Content:** **Question 18 (12 points)** **Question Stem:** In the quadrilateral pyramid P-ABCD, PD is perpendicular to the base ABCD, CD is parallel to AB, AD = DC = CB = 1, AB = 2, DP = √3. **Sub-questions:** (1) Prove: BD ⊥ PA; (2) Find the sine of the angle between PD and plane PAB. **Chart/Diagram Description:** * **Type:** Three-dimensional geometric figure, a quadrilateral pyramid. * **Main Elements:** * The pyramid is denoted as P-ABCD, with P as the apex and ABCD as the base. * **Vertices:** Labeled P, A, B, C, D. * **Edges:** * Visible (solid lines): PA, PB, PC, PD, AB, BC. * Hidden (dashed lines): AD, DC, DB (diagonal of the base). * **Base:** A quadrilateral ABCD. Based on the problem description (CD // AB, AD=DC=CB=1, AB=2), the base is a trapezoid. * **Apex:** P is positioned above the base. * **Perpendicularity:** The edge PD is drawn vertically, indicating its perpendicularity to the base ABCD as stated in the problem description. * **Overall appearance:** The figure depicts a pyramid with a non-rectangular base, viewed from an angle that shows faces PAB, PBC, PCD, PDA and the base ABCD.