帮我解释一下这道题---**Textual Information:** 如右图, 简单组合体 ABCDPЕ, 其底面 ABCD 为正方形, PD ⊥ 平面 ABCD, EC // PD, 且 PD = 2EC. (1) 若 N 为线段 PB 的中点, 求证: EN ⊥ 平面 PDB; (2) 若 $\frac{PD}{AD} = \sqrt{2}$, 求平面 PBE 与平面 ABCD 所成的锐二面角的大小. **Chart/Diagram Description:** * **Type:** 3D geometric figure. * **Main Elements:** * **Points:** A, B, C, D, P, E, N. * **Lines:** Solid lines: AB, BC, CP, PE, EB, AP, BP. Dashed lines: AD, DC, PD, AC, BD, DN, NE. * **Shapes:** The base ABCD is depicted as a square (although drawn in perspective as a parallelogram). The figure represents a combination of solids involving points P and E above the base. PD is shown as a vertical edge from P to D on the base. EC is a vertical edge from E to C on the base (parallel to PD). * **Planes:** The base plane is ABCD. Other visible planes include PAD, PCD, PBC, PAB, PDCB, PEBC, PEB, NEP, NEB, NDP, NDB. The problem mentions planes PDB, PBE, and ABCD. * **Labels:** Points A, B, C, D, P, E, N are labeled. * **Relative Position and Direction:** Point P is positioned such that PD is perpendicular to the base ABCD. Point E is positioned such that EC is parallel to PD and E is above C. N is located on the line segment PB. * **Annotations:** The diagram visually represents the geometric object described in the problem statement. The dashed lines indicate edges that are hidden from view or internal lines (like diagonals or line segment N). N is indicated as the midpoint of PB in the problem description and is drawn approximately in the middle of PB.