Đáp án + Giải thích các bước giải:

`1) x^2 + 2x.3 + ....`

`= x^2 + 2x . 3 + 3^2`

`= x^2 + 6x + 9`

`= (x + 3)^2`

`2) x^2 + 6xy + ...`

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

`= x^2 + 6x + 9y^2`

`= (x + 3y)^2`

`3) 4x^2 + .... + 81y^2`

`= (2x)^2 + 2.2x . 9y + (9y)^2`

`= (2x + 9y)^2`

`4) 1/4x^2 + ... + 4y^2`

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

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

`5) ..... + xy + 1`

`= (1/2xy)^2 + 2 . 1/2xy . 1 + 1^2`

`= (1/2xy + 1)^2`

`6) ....... + 100xy^2 + 25x^2`

`= (10y^2)^2 - 2 . 2y^2 . 5x + (5x)^2`

`= (10y^2 - 5x)^2`