Bài `1:` 

`a)` `x^6-8y^3`

`=(x^2)^3-(2y)^3`

`=(x^2-2y)[(x^2)^2+x^2.2y+(2y)^2]`

 `=(x^2-2y)(x^4+2xy^2+4y^2)`

`b)` `8/27 x^3-y^6`

`=(2/3 x)^3-(y^2)^3`

`=(2/3 x-y)[(2/3 x)^2+2/3 x.y^2+(y^2)^2]`

`=(2/3 x-y)(4/9x^2+2/3 xy^2+y^4)`

`c)` `343/8 x^3+27/64 y^3`

`=(7/2 x)^3+(3/4 y)^3`

`=(7/2 x+3/4 y)[(7/2 x)^2-7/2 x.3/4 y+(3/4 y)^2]`

`=(7/2 x+3/4 y)(49/2 x^2-21/8 xy+9/16 y^2)`

`d)` `216/27 a^3+125/8 b^3`

`=(2a)^3+(5/2 b)^3`

`=(2a+5/2 b)[(2a)^2-2a.5/2 b+(5/2 b)^2]`

`=(2a+5/2 b)(4a^2-5ab+25/4 b^2)`

Bài `2:`

`a)` `(2x+5y)(4x^2-10xy+25y^2)`

`=(2x)^3+(5y)^3`

`=8x^3+125y^3`

`b)` `(1/2 x+3y)(1/4 x^3-3/2 xy+9y^2)`

`=(1/2 x)^3+(3y)^3`

`=1/8 x^3+27y^3`

`c)` `(2x-3)(3x^2+6x+9)`

`=(2x)^3-3^3`

`=8x^3-27`

`d)` `(x-5y)(x^2+5xy+25y^2)`

`=x^3-(5y)^3`

`=x^3-125y^3`

`-----`

Tổng 2 lập phương: `a^3+b^3=(a+b)(a^2-ab+b^2)`

Hiệu 2 lập phuong: `a^3-b^3=(a-b)(a^2+ab+b^2)`