“MN是经过椭圆 （a＞b＞0）的焦点的任一弦，若过椭圆中心O的半弦 ，则 .”类比椭圆的性质，可得“MN是经过双曲线 （a＞0，b＞0）的焦点的任一弦(交于同支)，若过双曲线中心O的半弦 ，则____.”

                                                                                    A Nice Clean Plate
   It's Saturday evening. Bruce is having a birthday party. There are a lot of children at the party. They play together for an hour. Then Bruce's mother gives them some food. But she forgets to give Sam any.
   Sam feels wronged(委屈) but then he remembers his mother's words, “Be polite.(礼貌). Don't ask for food.” So he waits for a few minutes, then he holds his plate up over his head and says loudly, “ Does anyone want a nice clean plate?”