请帮我解答图片里面的试题，并生成解析视频---15. 如图，在四棱锥 P - ABCD 中， $AD \parallel BC, PA = BC = 2AD = 2AB = 4, AD \perp$ 平面 $PAB, PA \perp AB, E、F$ 分别是棱 $PB,PC$ 的中点 (1) 证明：$DF \parallel$ 平面 $ACE$； (2) 求平面 $ACE$ 与平面 $PCD$ 的夹角的余弦值. **Chart/Diagram Description:** * **Type:** Geometric figure of a quadrangular pyramid P-ABCD. * **Main Elements:** * **Vertices:** P (apex), A, B, C, D (base), E (on PB), F (on PC). * **Base:** A quadrilateral ABCD. Edges AB and BC are visible, AD and CD are dashed (hidden). * **Edges from Apex:** PA, PB, PC, PD. PA, PB, PC are solid (visible), PD is dashed (hidden). * **Lines within solid/faces:** AE and CE are dashed lines. DF and EF are solid lines. * **Labels:** Vertices are labeled P, A, B, C, D, E, F. * **Line Styles:** Solid lines represent visible edges/lines (PA, PB, PC, BC, DF, EF, AB). Dashed lines represent hidden edges/lines or lines within the solid (PD, AD, CD, AE, CE). * **Relative Position:** P is above the base ABCD. E is on PB, F is on PC. DF is a line segment connecting vertex D to point F on PC. AE connects A to E on PB, CE connects C to E on PB. The plane ACE is suggested by lines AE and CE.