A:to blame; occurred
B:to blame; occurring
C:to be blamed; occurred
D:to be blamed; occurring
(Ⅰ)求直方图中a的值;
(Ⅱ)设该市有30万居民,估计全市居民中月均用水量不低于3吨的人数,并说明理由;
(Ⅲ)估计居民月均水量的中位数.
在匀强电场中,下列说法正确的是( )
如图,AB=AC,BD=DC,DE⊥AB,DF⊥AC,垂足分别是E,F.求证:DE=DF.