给我讲一下这个题目---**Textual Information:** **Question Stem:** 如图, 菱形ABCD的边长是18, 如果三角形CDE是等腰直角三角形, 求四边形ABEF的面积. **Mathematical Formulas/Equations:** None explicitly written as formulas, but the problem involves calculating area based on given lengths and geometric properties. **Other Relevant Text:** "如图" (As shown in the figure) "菱形ABCD的边长是18" (The side length of rhombus ABCD is 18) "三角形CDE是等腰直角三角形" (Triangle CDE is an isosceles right-angled triangle) "求四边形ABEF的面积" (Find the area of quadrilateral ABEF) **Options:** None. **Chart/Diagram Description:** * **Type:** Geometric diagram. * **Main Elements:** * **Shapes:** A rhombus ABCD is shown. A triangle CDE is attached to vertex C. * **Points:** Points A, B, C, D, E, and F are labeled. * **Lines:** The boundaries of the rhombus are formed by line segments AB, BC, CD, and DA. The triangle CDE is formed by line segments CD, DE, and CE. A diagonal AC of the rhombus is drawn. The line segment DE intersects the diagonal AC at point F. There is also a line segment connecting B and E, forming part of the quadrilateral ABEF. * **Angles:** A square symbol with a dot indicates a right angle at vertex E of triangle CDE. * **Labels:** Vertices of the rhombus are labeled A, B, C, D in counterclockwise order starting from the left. The third vertex of the triangle is labeled E. The intersection of AC and DE is labeled F. * **Relative Position and Direction:** Point E is positioned below the diagonal AC and to the right of vertex C. The triangle CDE is attached to vertex C of the rhombus. Point F lies on the diagonal AC and the segment DE. Quadrilateral ABEF is formed by vertices A, B, E, and F. * **Legend:** None.