题目

   Qingdao lies in the east of Shandong Province. As we all know, it is an important port city and it is an economic and   61   (culture) center. It is also home to the Tsingtao Brewery and the  62  (large) international beer festival in China. Of course Qingdao is a very beautiful city, too. Many places around the city are well worth   63   (visit). Every year, the festival   64   (hold) on schedule. Millions of visitors   65   come from the world go to the city during the festival.    It is very convenient to go to Qingdao. It is   66   (rough) 560 kilometer 已知点P(a，b)在x、y轴上的射影分别为点A、B， (1)求直线AB的方程； (2)求过点P且平行于AB的直线l的方程； (3)求过点P且垂直于AB的直线m的方程．