说明：  三点画圆的程序，展示VB中Line和Shape控件的使用例子，随意点击窗体三下可以画出一个圆，右键菜单可以显示/隐藏线条等
(Draw Circle By Three Point Program, the example shows the useage of Line and Shape controls in VB , click the Form 3 times may be arbitrarily draw a circle, right-click menu can show/hide the lines, etc.)