这道题怎么做，其中包含的知识点---四、解答题 15. 如图, 在平行六面体 ABCD-A'B'C'D' 中, AB=AD=AA'=1, ∠A'AB=∠A'AD=60°, ∠DAB=90°, 点 O 为 BC' 的中点, $\vec{AO} = x\vec{AB} + y\vec{AD} + z\vec{AA'}$. (1) 求 x+y+z 的值; (2) 求 AO 与 DC 所成角的余弦值. **图描述 (Figure Description for Question 15):** - Type: 3D geometric figure, a parallelepiped ABCD-A'B'C'D'. - Vertices: Labeled A, B, C, D on the bottom face and A', B', C', D' on the top face. A' is above A, B' above B, C' above C, and D' above D. - Edges: Solid lines show visible edges (AB, BC, CD, BB', CC', DD', A'B', B'C', C'D'). Dashed lines show hidden edges (AD, DA', DD'). - Point O: Marked as the midpoint of the edge BC', which is a top edge. - Line Segment: A dashed line segment AO is drawn from vertex A to point O. - The figure depicts a parallelepiped with base ABCD and top face A'B'C'D'. 16. 如图, 四棱锥 P-ABCD 的侧面 PAD 是边长为2的正三角形,