据说 ,孙悟空用金箍棒在地面上画了一个半径为Ｒ的圆而携棒离去后 ,余下的师徒三人不仅在该圆之内能避妖袭 ,而且在以该圆的任一弦为直径的圆内也同样安然无恙 .试问这个安全区域的面积有多大 ?笔者组织学生讨论 ,求得问题的答案 ,并由此探索出一些结果 .如图 ,设悟空所画之圆? It is said that after the Sun Wukong painted a circle of radius R on the ground with a golden cudgel and left with the stick to leave, the remaining men and women of the apprentice could not only avoid the raiding within the circle, but also take any of the circles. The circle within the diameter of the string is equally safe. How large is the area of ​​the safe area? The author organizes students to discuss and obtain answers to the questions, and from this finds out some results. As shown in the figure, set the circle painted by Goku?