题目

Societies all over the world name places in similar ways. Quite often there is no official naming ceremony but places tend to be called names as points of reference by people. Then an organized body steps in and gives the place a name. Frequently it happens that a place has two names: One is named by the people and the other by the government. As in many areas, old habits die hard, and the place continues to be called by its unofficial name long after the meaning is lost.Many roads and places in Singapore(新加坡) are named in order that the pioneers will be remembered by future generations.如图,在梯形ABCD中,AD∥BC,∠B=90°,AD=18cm,BC=21cm,点P从点A开始沿AD边向D以1cm/s的速度移动,点Q从点C开始沿CB边向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t(s),求:(1)t为何值时,四边形PQCD为平行四边形;(2)t为何值时,四边形ABQP为矩形;(3)t为何值时,梯形PQCD是等腰梯形。
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