一个梯形的上底与下底的和是20cm，梯形的高是8cm，面积是______cm2．
20×8÷2，
=160÷2，
=80（平方厘米）；
故答案为：80．
Just _____ at the time! I _____ no idea it was so late.[ ]
A. look, have
B. looking, had
C. look, had
D. looking, have
对立井防爆门的要求有（）
A.面积不小于出风井口的面积
B.正对出风井风流方向
C.应该设置2道
D.严密、不漏风
E.挂平衡锤
