tính;A.(x-2y)^2; B.(x-2)(x^2+2x+4);C.(2x-1)^3

`**)` Bài tập áp dụng `2` đẳng thức:

`1) (x – y)^2 = x^2 – 2xy + y^2`

`2) (x – y)^3 = x^3 – 3x^2y + 3xy^2 – y^3`
Giải:

`a_)`

`(x – 2y)^2`
`= x^2 – 2x. 2y + (2y)^2`

`= x^2 – 4xy + 4y^2`

`b_)`

`(x – 2)(x^2 + 2x + 4)`

`= x. x^2 + x. 2x + x. 4 – 2. x^2 – 2. 2x – 2. 4`

`= x^3 + 2x^2 + 4x – 2x^2 – 4x – 8`

`= x^3 + (2x^2 – 2x^2) + (4x – 4x) – 8`

`= x^3 – 8`

`c_)`

`(2x – 1)^3`
`= (2x)^3 – 3. (2x)^2. 1 + 3. 2x. 1^2 – 1^3`

`= 8x^3 – 3. 4x^2 + 6x – 1`

`= 8x^3 – 12x^2 + 6x – 1`