已知圆心为C的圆经过点 A（1，1）和B（2，﹣2），且圆心C在 直线L：x﹣y+1=0上，求圆C的标准方程．

point   

    Anna is a Chinese girl . She is ten . She is in London with her parents. She doesn't know much English. She can't speak English well. Sometimes her friends don't understand her .

    It's Sunday morning .She is going to the zoo to see the panda . She is waiting at a bus stop .At the bus stop she asks a boy how to go to the zoo. But the boy can't understand her .Then she takes out a pen and some paper. She draw a panda on it and shows the picture to the boy .The boy smiles and then shows her the way to the zoo.