∵|2x-y+5|+(x+2y-4)的平方=0
∴2x-y+5=0,
x+2y-4=0
∴2x-y=-5,
x+2y=4
x-3y=-9
则（2x-y)的三次方-（x-3y)(y-2x)的平方
=(-5)^3-(y-2x)²(x-3y)
=-125-16*(-9)
=19