题目

阅读理解。 Kate is a little girl. She’s only five. She is not at school. She doesn’t know how to read or write. But her sister Mary is a student. She knows how to read and write. One day, Mary sees her little sister in the room. She is at the table(桌子). There is(有) a pencil in her hand. She is writing. “What are you writing, Kate?” she asks. “I’m writing a letter to my friend, Rose.” says Kate. “But how can you? You don’t know how to write.” says her sister. “Well.” says Kate, “It doesn’t matter(不要紧). Rose doesn’t know how to read, either(如图，平面直角坐标系中，已知点，若对于平面内一点C，当是以AB为腰的等腰三角形时，称点C时线段AB的“等长点”．请判断点，点是否是线段AB的“等长点”，并说明理由；若点是线段AB的“等长点”，且，求m和n的值．