解答这个题目，生成详细的讲解视频---```plain text 1. 本小题满分 12 分) (注意: 在试题卷上作答无效) 如图, 四棱锥 S-ABCD 中, SD⊥底面 ABCD, AB//DC, AD⊥DC, AB=AD=1, DC=SD=2, E 为棱 SB 上的一点, 平面 EDC⊥平面 SBC . (I) 证明: SE=2EB; (II) 求二面角 A-DE-C 的大小. Chart Description: Type: Geometric figure (Pyramid). Main Elements: - Vertices: S (apex), A, B, C, D (base), E (on edge SB). - Base ABCD is a quadrilateral. - SD is shown as the height from S to the base, represented by a dashed vertical line. - Edges SA, SB, SC, AB, BC, CD, DA. Edges SD, AD, DC, AB are partially or fully dashed. - Point E is on edge SB. - Lines DE and EC are drawn, forming plane EDC. - The figure represents a right pyramid with a quadrilateral base. - The text describes the base ABCD as a trapezoid with AB parallel to DC and AD perpendicular to DC. Given lengths are AB=AD=1, DC=2, and height SD=2. E is a point on SB. - The diagram visually shows the spatial arrangement of the points and edges described. ```