题目

（本题满分10分）如图，在梯形中，两点在边上，且四边形是平行四边形．（1）与有何等量关系？请说明理由；（2）当时，求证：是矩形．  答案：（1）解：．························································································ （1分）理由如下：，四边形和四边形都是平行四边形.．又四边形是平行四边形，．．．·············································�函数f(x)= (a>0,a≠1)的图象恒过定点（   ）. A．            B．            C．            D．