lx-1l+lx+3l有最小值当x≤-3 lx-1l+lx+3l=1-x-x-3=-2x-2 x=-3时有最小值4当-3<x≤1 lx-1l+lx+3l=1-x+x+3=4当x>1 lx-1l+lx+3l=2x+2 恒大于4总上 当-3≤x≤1时 lx-1l+lx+3l有最小值4