1÷5=____ 4÷12-=____
菱形ABCD的一条对角线长为6，边AB的长为方程y2﹣7y+10=0的一个根，则菱形ABCD的周长为（ ）
Because he is the tallest in his class, __________ he sits at the back of the classroom.
黄河水土流失最严重的河段是( )
