根据上面的题目内容，生成微课视频---Here is the extraction of the content from the image: **Problem Description:** 如图, 在四棱锥 $P-ABCD$ 中, 底面 $ABCD$ 为矩形, $PA \perp$ 底面 $ABCD$, $AP = AD = 4$, $AB = 3$, 点 $M$ 为 $PD$ 的中点, 连接. **Geometric Figure Description:** * **Type:** 3D geometric figure, specifically a pyramid with a rectangular base. * **Vertices:** Labeled as P (apex), A, B, C, D (base vertices). M is a point on edge PD. * **Base:** ABCD is a rectangle. Edges AB, BC, CD, DA are shown, with AB and CD appearing parallel, and AD and BC appearing parallel. Angles at the vertices A, B, C, D are right angles. * **Edges:** * Edges PA, PB, PC, PD connect the apex P to the base vertices. PA is shown as a solid line and appears perpendicular to the base. PB, PC, PD are shown as solid lines. * Edges of the base AB, BC, CD, DA are shown. AB and AD are solid lines. BC and CD are solid lines. * **Dashed Lines:** Lines PA, AC, and BD within the pyramid are shown as dashed lines, indicating they might be hidden or construction lines. However, the description states PA is perpendicular to the base and the diagram shows PA as a solid line from P downwards to A. The dashed line segment seems to extend from P directly downwards to the point A on the base. The line from A to C is dashed, representing a diagonal of the base. The line from B to D is also dashed, representing the other diagonal of the base. The dashed line segment within the pyramid from P down to A appears to represent the height. Let's assume the point labeled A on the base is directly below P. Looking closely, the line segment from P directly down to the base (at A) is dashed, and the segment PA is solid. The point labeled A is one of the base vertices. The dashed line from P appears to be the altitude dropped from P to the base at point A. So PA is the edge connecting P to A, and the altitude is PA. The diagram shows PA as a solid edge. The dashed line from P vertically downwards inside the figure is likely an indication of the height or perpendicularity, but its endpoint is not explicitly labeled. Let's interpret the dashed lines as hidden edges or construction lines as is standard in geometry diagrams. PA is a solid edge. AB, AD, BC, CD are solid edges of the base. PB, PC, PD are solid edges. AC and BD are dashed diagonals of the base. * **Point M:** Labeled on edge PD, described as the midpoint of PD. * **Connections:** Line segment AM is drawn as a solid line connecting A to M. Line segment BM is drawn as a solid line connecting B to M. Line segment CM is drawn as a solid line connecting C to M. * **Relative Position:** P is above the base ABCD. A, B, C, D form a rectangle in the base plane. M is on the edge PD. PA is perpendicular to the base ABCD. **Given Information:** * Shape: Quadrangular pyramid P-ABCD. * Base: ABCD is a rectangle. * Perpendicularity: $PA \perp$ base $ABCD$. * Lengths: $AP = AD = 4$, $AB = 3$. * Point M: M is the midpoint of PD. **Questions:** (1) 求三棱锥 $M-ACD$ 的体积; (Find the volume of the triangular pyramid M-ACD;) (2) 求异面直线 与 $PB$ 所成的角的余弦值. (Find the cosine of the angle between the skew lines and PB.) **Other Relevant Text:** (In red text below the questions, partially visible) (1) 过程 MH//AD. This seems to be a hint or part of a solution related to question (1), suggesting using a line MH parallel to AD. The rest of the line is cut off.