http://www.baidu.com/s?wd=%B7%D6%CE%F6%C3%C0%D1%A7%D3%EB%C8%CB%B8%F1%C0%ED%C2%DB&lm=0&si=&rn=10&ie=gb2312&ct=0&cl=3&f=1&rsp=6你的问题有难度是作业吗?