如何解答这道题---**Extracted Content:** **Question Stem:** 18. 如图, 点 A, B, C, D 在 ⊙O 上, O 点在∠D 的内部, 四边形 OABC 为平行四边形, 求∠OAD+∠OCD 的度数. (Translation: As shown in the figure, points A, B, C, D are on circle ⊙O. Point O is inside angle ∠D. Quadrilateral OABC is a parallelogram. Find the degree measure of ∠OAD+∠OCD.) **Chart/Diagram Description:** * **Type:** Geometric figure, specifically a circle with several points and lines. * **Main Elements:** * **Circle:** A circle is drawn with its center labeled as O. * **Points:** * Point O is the center of the circle. * Points A, B, C, D are located on the circumference of the circle. * **Lines/Segments:** * Segments OA, OB, OC, OD connect the center O to points A, B, C, D on the circumference, respectively. These are all radii of the circle. * Segments AB, BC, CD, DA connect points on the circumference. * Segments OA, AB, BC, CO form the quadrilateral OABC. * Segments DA, DC, CO, OA form the quadrilateral OADC. * Segments DA, DO, OA form triangle DAO. * Segments CD, CO, OD form triangle CDO. * **Shapes:** * A circle ⊙O. * A quadrilateral OABC. * A quadrilateral ABCD (an inscribed quadrilateral). * Triangles OAD and OCD. * **Labels and Annotations:** * Vertices are labeled A, B, C, D, O. * There is a single arc mark inside ∠OAD at vertex A. * There is a single arc mark inside ∠OCD at vertex C. These marks typically indicate angles, and if identical, might suggest equality of the angles they mark, though this is not explicitly stated.