`14)Q=(5x-2y)^2-(x+3y)^3+(2x+y)^2+(x-2y)^3`
`=(25x^2-20xy+4y^2)-(x^3+9x^2y+27xy^2+27y^3)+(4x^2+8xy+y^2)+(x^3-6x^2y+12xy^2-8y^3)`
`=25x^2-20xy+4y^2-x^3-9x^2y-27xy^2-27y^3+4x^2+8xy+y^2+x^3-6x^2y+12xy^2-8y^3`
`=(25x^2+4x^2)+(-20xy+8xy)+(4y^2+y^2)+(-x^3+x^3)+(-9x^2y-6x^2y)+(-27xy^2+12xy^2)+(27y^3-8y^3)`
`=29x^2-12xy+5y^2-15x^2y-15xy^2+19y^3`
Thế `x=1/5;y=-1/3` vào $Q$ ta có:
`Q=29×(1/5)^2-12×1/5×(-1/3)+5×(-1/3)^2-15×(1/5)^2×(-1/3)+19×(-1/3)^3`
`=29/25+4/5+5/9+1/5-19/27`
`=1358/675`
$\text{@ThanhTruc2008}$
