《四书》指的是____、____、____和____。他得名于宋朝的____。
There is a sofa (沙发)in this room. We chat (聊天)with each other and watch TV in this room. It's a____.
已知如图,D是△ABC中AB边上的中点,△ACE和△BCF分别是以AC、BC为斜边的等腰直角三角形,连接DE、DF.
求证:DE=DF.