f(x)=|2x+1|-|x-3|x≤-1/2时：f(x)=-2x-1+x-3=-x-4单调减。f(-1/2)=0-1/2-3=-7/2-1/2＜x＜3时：f(x)=2x+1+x-3=3x-2单调增，f(-1/2)=3*(-1/2)-2=-7/2x≥3时：f(x)=2x+1-x+3=x+4，单调增∴ymin=f(-1/2)=-7/2...
