Đáp án:
`1a,=-x^2+3x-1`
`b,=x^3-9x^2y-27xy^2-27y^3+21x^2-24xy+3y^2`
`2,A=-x^3-8y^3+18x^2-6xy+y^2`
Giải thích các bước giải:
`1a,(x-3)^3-(x-2)^3`
`=(x-3-x+2)[(x-3)^2+(x-3)(x-2)+(x-2)^2]`
`=-(x-3)^2-(x-3)(x-2)-(x-2)^2`
`=x^2-6x+9-x^2+5x-6-x^2+4x-4`
`=-x^2+3x-1`
`b,(5x-2y)^2-(x+3y)^3-(2x+y)^2`
`=25x^2-20xy+4y^2-x^3-9x^2y-27xy^2-27y^3-4x^2-4xy-y^2`
`=x^3-9x^2y-27xy^2-27y^3+21x^2-24xy+3y^2`
`2,A=(3x-y)^2-(x+2y)*(x^2-2xy+4y^2)+(3x)^2`
`=9x^2-6xy+y^2-x^3-8y^3+9x^2`
`=-x^3-8y^3+18x^2-6xy+y^2`
