`A=(x+3)(x^2-3x+9)-(54+x^3)`
`A=x^3+27-54-x^3`
`A=(x^3-x^3)+27-54`
`A=0-27=-27.`
`B=(2x+y)(4x^2-2xy+y^2)-(2x-y)(4x^2+2xy+y^2)`
`B=8x^3+y^3-(8x^2-y^3)`
`B=8x^3+y^3-8x^3+y^3`
`B= (8x^3-8x^3)+(y^3+y^3)`
`B= 2y^3.`

`C=(a+b)^2-(a-b)^2`

`C=a^2+2ab+b^2-(a^2-2ab+b^2)`

`C=a^2+2ab+b^2-a^2+2ab-b^2`

`C=(a^2-a^2)+(b^2-b^2)+(2ab+2ab)`

`C=4ab.`

`D=(a+b)^3-(a-b)^3-2b^3`
`D=a^3+3a^2b+3ab^2+b^3-(a^3-3a^2b+3ab^2-b^3)-2b^3`

`D=a^3+3a^2b+3ab^2+b^3-a^3+3a^2b-3ab^2+b^3-2b^3`
`D=(a^2-a^2)+3a^2b+3a^2b+3ab^2-3ab^2+(b^3+b^3-2b^3)`
`D=6a^2b.`
`E=(x+y+z)^2-2(x+y+z)(x+y)+(x+y)^2`
`E=(x+y+z-x-y)^2`
`E=z^2.`

Lưu ý: Phần `E` chắc bạn chép sai đề. Xem lại nha.  

`A = (x + 3)(x^2 - 3x + 9) - (54 + x^3)`

`A = x^3 + 27 - 54 - x^3`

`A = -27`

`B = (2x + y)(4x^2 - 2xy + y^2) - (2x - y)(4x^2 + 2xy + y^2)`

`B = 8x^3 + y^3 - 8x^3 + y^3`

`B = 2y^3`

`C = (a + b)^2 - (a - b)^2`

`C = (a + b - a + b)(a + b + a - b)`

`C = 2b.2a`

`C = 4ab`

`D = (a + b)^3 - (a - b)^3 - 2b^3`

`D = (a + b - a + b)[(a + b)^2 + (a + b)(a - b) + (a - b)^2] - 2b^3`

`D = 2b . (a^2 + 2ab + b^2 + a^2 - b^2 + a^2 - 2ab + b^2) - 2b^3`

`D = 2b . (3a^2 + b^2) - 2b^3`

`D = 6a^2b + 2b^3 - 2b^3`

`D = 6a^2b`

`E = (x + y + z)^2 - 2(x + y + z)(x + y) + (x + y)^2`

`E = (x + y + z - x - y)^2`

`E = z^2`

$\text{câu E hình như thiếu mũ 2 thì phải :v nếu k có mũ hai thì lẻ lắm :3}$