Đáp án:
a, `(9x^2 + 12x + 4) + 6(3x + 2) + 9`
` = 9x^2 + 12x + 4 + 18x + 12 + 9`
` = 9x^2 + 30x + 25`
` = (3x)^2 + 2 . 3 . 5x + 5^2`
` = (3x + 5)^2`
b, ` 9x^2 + 4y^2 + 2(3x + 2y + 6xy) + 1`
` = 9x^2 + 4y^2 + 6x + 4y + 12xy + 1`
` = (3x)^2 + (2y^2) + 2x . 3 + 2y . 2 + 1^2`
` =(3x + 2y + 1)^2`