题目

Beep... beep... There goes the bell! Robbie opens his eyes. He sits in the room for a whole day and now it is time for him to do something.Robbie looks out of the window. It is still snowing heavily and there is ice on the window. It is another cold day. Robbie is told to turn the heat on before the family get home. And he does it.Then Robbie is told to do some cleaning work at once. It is an easy job for him but a tough one for his master Helen. He keeps on working until every room is clean and tidy.For now he has to cook supper for the family.The first thing Robbie does is to get a big pot ( 在圆O中，弦AB的长为6，它所对应的弦心距为4，那么半径OA＝________．