I have a rule for travel: Never carry a map. I prefer to ask for directions. Foreign visitors are often puzzled in Japan because most streets there don’t have names. In Japan, people use landmarks i
(08·福建D篇) The global energy crisis is approaching.What can we do?Here are some steps you can take. Cooling puts the greatest stress on your summer energy bill and the power grid（电网）
满足什么条件时，
：
，
：
，
：
三条直线只有两个交点？
时，三条直线有两个交点
时，
的方程均为
，即三条直线重合；
，
，
，
且
与
都和
相交．