题目

  This year, Sichuan schools decide to start morning classes an hour later. This will help students feel less tired. And most junior high schools will begin the first class at about 8:30 am.    A. Wu Hongli feels the same. She said the new timetable (时间表) was good for her health. “I used to get up at 6:30 am. I had to drink coffee every night, or I would feel too sleepy to finish my homework,” said Wu. “Now I can finish it without coffee. Because I can sleep a little longer.”   B. The students are happy about the change. Liu Ming told us that his class were excited by the new如图所示,梯形ABCD中,AD∥BC,∠B=90°,AB=14cm,AD=18cm,BC=21cm.点P 从点A开始沿AD边向点D以1cm/s的速度移动.点Q从点C开始沿CB向点B以2cm/s 的速度移动,如果P、Q分别从A、C同时出发,设移动的时间为ts,问t为何值时,梯形PQCD 是等腰梯形?