解答下该题的三角函数问题---**Textual Information:** 如图, 四边形 ABCD 中, AB//CD, ∠DAB = 90°, F为CD中点, E在AB上, EF//AD, AB = 3AD, CD = 2AD. 将四边形 EFDA 沿 EF 翻折至四边形 EFD'A', 使得面 EFD'A' 与面 EFCB 所成的二面角为 60°. (1) 证明: A'B // 平面 CDF. (2) 求面 BCD' 与面 EFD'A' 所成二面角的正弦值. **Chart/Diagram Description:** * **Type:** 3D geometric figure illustrating a solid formed by folding a plane figure. * **Main Elements:** * **Points:** Labeled points include A, B, C, D, E, F, A', D'. Points A and D are shown in their original planar positions (dashed) and their new positions after folding (A' and D'). * **Lines:** Lines are represented as solid or dashed segments connecting points. Dashed lines appear to represent the original planar configuration (ABCD, EFDA). Solid lines represent the folded 3D configuration (EFD'A', EFCB, and connections like A'B, BC, BD', CD'). The fold line is EF. * **Shapes:** * The original figure is a quadrilateral ABCD, shown with AB parallel to CD and angle DAB being 90 degrees. This indicates it's a trapezoid or rectangle. * EFDA is a part of ABCD, seemingly a rectangle or square based on EF//AD and ∠DAB=90°. * EFD'A' is the position of EFDA after folding along EF. It's shown inclined relative to the original plane. * EFCB is the remaining part of the original figure, which lies in the original plane. * **Relative Position and Direction:** * F is the midpoint of CD. * E is on AB. * EF is parallel to AD. * The quadrilateral EFDA is folded along EF. * A' and D' are the positions of A and D after folding. * The dihedral angle between plane EFD'A' and plane EFCB is 60 degrees. * The original points A and D are shown with dashed lines connecting to their folded positions A' and D' and also forming the original figure ABCD. * The points A, B, C, D, E, F form the initial planar configuration. After folding, points A and D move to A' and D', creating the 3D configuration with planes EFD'A', EFCB, BCD', etc.