根据短文内容填空。
This is Mike's bedroom. It's new and nice. Today Mike's friends, Tom and Lucy are here, too. They are playing video games. Many things are on the bed. It's messy（凌乱的）. The coat is Mike's. The T-shirt and the socks are Tom's. There is a red cap. It must be Lucy's.
过圆
的圆心,则a的值为 （ ）