分解因式:a2-2a(b+c)+(b+c)2.
a2-2a(b+c)+(b+c)2=(a-b-c)2.
It seems water ______ from this tap for some time. We’ll have to take it apart to put it right.
A.had leaked
B.is leaking
C.leaked
D.has been leaking
什么叫工作接地和保护接地?又什么叫保护接零?为什么同一低压配电系统中不能有的设备采取保护接地,有的设备又采取保护接零?