立体几何中 ,正方体与球的组合问题是一种常见题 ,但由于学生常弄不清是球内切于正方体还是球与正方体的棱相切 ,因此解这类问题时常出错 ,例如下面的例题是学生很容易做错的一道题 .例　有一个棱长为a的正方体骨架 ,其内放置一个气球 ,使其充气且尽可能地膨胀(仍保持为球的形? In the three-dimensional geometry, the combination of cube and ball is a common problem, but because students often do not understand whether the ball is tangent to the cube or the sphere is tangent to the cube, solving this type of problem often occurs, such as the following example. It is a question that students can easily make a mistake. For example, there is a cube with an a-squared prism and a balloon is placed inside it to inflate it and expand it as much as possible (still keeping the ball shape?