生成这道题的解题思路和步骤---**Question Stem:** 如图，已知正方形 ABCD 的边长为 6，圆 B 的半径为 3，点 P 是圆 B 上的一个动点，则 PD - (1/2) PC 的最大值为 **Mathematical Expression:** PD - (1/2) PC **Given Information:** 正方形 ABCD 的边长为 6 圆 B 的半径为 3 点 P 是圆 B 上的一个动点 **Diagram Description:** * **Type:** Geometric figure showing a square and a circle. * **Main Elements:** * A square labeled ABCD. The vertices are labeled A, B, C, and D in a counter-clockwise order starting from the top-left corner. * A circle centered at point B. * A point P is shown on the circumference of the circle. * Line segments PD and PC connect point P to vertices D and C of the square, respectively. * **Relative Position and Direction:** * B is the bottom-left vertex of the square and the center of the circle. * The circle is located such that its center is at B. Since the side length of the square is 6 and the radius of the circle is 3, the circle passes through points on AB and BC at a distance of 3 from B. * P is shown as a point on the circle. **Other Relevant Text:** (No other relevant text besides the question and watermarks)