Đáp án + Giải thích các bước giải:

`Bài  21:`

`a. A = (2x + y)^2 - (2x - y)^2`

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

`= [ 2x + y - 2x + y] . [2x + y + 2x - y]`

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

`= 2y . 4x`

`= 8xy`

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

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

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

`= [x - 2y - 2y]^2`

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

`= (x - 4y)^2`

`= x^2 - 8xy + 16y^2`

`Bài  22:`

`a. C = (x + y - z)^2`

`= x^2 + y^2 + z^2 + 2xy - 2yz - 2xz`

`b. D = (a + 1 - b)^2`

`= a^2 + 1^2 + b^2 + 2a - 2b - 2ab`

`= a^2 + 1 + b^2 + 2a - 2b - 2ab`

_________________________________________________________________

`A^2 - B^2 = (A - B)(A + B)`

`(A - B)^2 = A^2 - 2AB + B^2`

`(A + B - C)^2 = A^2 + B^2 + C^2 + 2AB - 2BC - 2AC`