They had received no news of him since his ____ (离开) from the island.
用配方法解方程x2-2x-5=0时,原方程应变形为( )
在硝酸铜和硝酸银的混合溶液中加入一定量的铁粉,充分反应后过滤,向滤出的固体中滴加稀盐酸,有气泡产生。下列判断正确的是