(2mx2-x2+3x+1)-(5x-4y+3x)
=2mx2-x2+3x+1-5x+4y-3x
=(2m-1)x2-5x+4y+1
不含x2项则2m-1=0
m=1/2
2m3-[3m3-（4m-5）+m]
=2m3-3m3+（4m-5）-m
=-m3+3m-5
=-1/8+3/2-5
=-29/8