A:从天下为公到天下为家
B:从诸侯争霸到中央集权
C:从刀耕火种到铁犁牛耕
D:从封邦建国到设置郡县
已知函数fx=-x2,x≥0x2+2x,x<0,则f(2)=____ ;不等式f(x)<3的解____
——Thank you so much for your present. It's really nice.——____________