求解答---**Text Extraction:** 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 EFDA', 使得面 EFDA' 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. **Diagram Description:** * **Type:** Geometric figure illustrating a 3D structure formed by folding a 2D shape. * **Elements:** * The initial state shows a quadrilateral ABCD with a line segment EF inside. Points A, B, C, D, E, F are labeled. The line segment EF appears to divide the quadrilateral into EFDA and EFCB. Dashed lines indicate parts of the shape that might be hidden or represent the original position. * The folded state shows a different arrangement of points A and D, denoted as A' and D'. The quadrilateral EFDA has been folded along EF to become EFDA'. Points B and C remain in their original plane relative to EF. * The points shown are A, B, C, D, E, F, A', D'. * Lines connect these points, forming quadrilaterals EFDA (partially shown in dashed lines) and EFCB, and the folded quadrilateral EFDA'. * Lines A-B, B-C, C-D (dashed), D-A (dashed), E-F, A'-E, A'-F, D'-E, D'-F, A'-D', A'-B, D'-C, B-C, E-B, F-C are depicted. * The diagram shows the original planar shape ABCD and the resulting 3D configuration after folding EFDA along EF. * Point E is on AB, F is on CD. EF is parallel to AD. * A' is the new position of A, and D' is the new position of D after folding. * A' is connected to E, F, B, and D'. * D' is connected to E, F, C, and A'. * The line A'B is shown as a solid line. * The quadrilateral CDF is implied by points C, D, F. The plane CDF is the plane containing these points. * The planes involved in the dihedral angles are labeled implicitly by the vertices defining them (EFD'A', EFCB, BCD). * Dashed lines seem to indicate original positions (A, D, CD, DA) and lines hidden from view or not part of the immediate structure (like CD, AD, AB). * Solid lines represent the visible parts of the folded structure (A'E, A'F, D'E, D'F, EFB, EFC, A'B, D'C, A'D'). * The structure resembles a truncated prism or a shape with a folded flap.