`5,`
`E = (2x-y)(4x^2+2xy+y^2)-(3x+y)^3+(x-2y)^3` $\text {tại}$ `x=-1` ; `y=2`
`E = 8x^3-y^3-27x^3-27x^2y-9xy^2-y^3+x^3-6x^2y+12xy^2-8y^3`
`E = -18x^3-10y^3-33x^2y+3xy^2`
$\text {→Thay}$ `x=-1` ; `y=2`
`E = -18.(-1)^3-10.2^3-33.(-1)^2. 2+3.(-1).2^2`
`E = -140`
$\text {@lamtung2007}$