Information is received into our brain by attention, and its quality determines (决定) how well it will be processed, saved and used afterwards.All information we receive is chosen by our attention.
A volcanic eruption in Iceland has sent ash across northern Europe．Airlines have stopped or changed the flights across the Atlantic Ocean, leaving airport hundreds of passengers stuck in airports．
是抛物线
的焦点，过
的直线交
于
两点．设
，则
的值等于 ． 
,与抛物线
联立得
, 求得交点的横坐标为
或
,∵
,∴