直流斩波器一般有（）。
CDE
暂无解析
求函数f(x)=x^3-3x^-9x+2的单调区间和极值.
Itisusefultoscience_______theearthasanobjectinspace.
You________gonow.It’sgettinglate.
Hethoroughlyunderstoodthesociety__________.
已知a,b为常数，时为等价无穷小，求a,b.
