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？
