（Ⅰ）求圆C的普通方程及直线l的直角坐标方程；

（Ⅱ）设直线l被圆C截得的弦长为 $2\sqrt{3}$ ，求m的值．

C

    Mrs. Tan was feeling nervous because her son Tom's exam was coming. This was his national exam and they did not know what to expect. Tom's weakest subject was Chinese. So Mrs Tan had found a private teacher to give him extra lessons to improve his Chinese. Tom, on the other hand, was not showing any interest in the subject. He thought that the extra lessons would not be helpful to his grades at all. So he didn't put much effort into the lesson.

    One day after Tom's Chinese lesson, his teacher spoke to Mrs. Tan. She said that Tom was showing a callous attitude (冷漠的态度) to his work. She was worried about that he might not pass his Chinese exam if he continued being like this. She had no idea that the situation was becoming so terrible. She told the teacher that she would have a word with Tom about it.

    That evening, both Mr. and Mrs. Tan sat down with Tom and talked to him. They wanted to know why his attitude was so bad. They managed not to scold him but instead, talked to him kindly. At first, Tom said nothing. Then, he slowly told them he did not understand his lessons. He said that the teacher's pace (速度) of lesson was too fast for him. Mrs. Tan then advised him to speak up if it happened the next time so that his teacher could slow down. In addition, Mrs. Tan would let his teacher know that she needed to slow down her lessons a little.

    After that heart-to-heart discussion with Tom, the situation improved. His teacher taught him at a pace that was comfortable to him and he in turn, would speak up and ask questions whenever he had problems.

    By the time Tom sat for his exam, he was feeling more confident. When his results came out, he was excited as he had improved a lot in Chinese. Both his parents as well as his private teacher were very proud of him. It goes to show that with an effort, all things are possible.

2016年是西安事变爆发80周年。小明同学在参观西安事变纪念馆后，写道：“一进入纪念馆，首先映入眼帘的是墙上方的五个大字‘历史的转折’。”（见下图）“历史的转折”是指（    ）