Đáp án:
$9x^2 +42x +49$
$ = (3x)^2 + 2 . 3 . 7 + 7^2 $
$ = (3x +7)^2$
$25x^2 -2xy +\dfrac{1}{25}y^2$
$ = (5x)^2 - 2 . 5x . \dfrac{1}{5}y +\dfrac{1}{25}y^2$
$ = (5x - \dfrac{1}{5}y)^2$
$27 + (x-3)(x^2+3x+9)$
$ = 27+x³-27$
$ =x³$
$x^3 +3x^2 +3x +1$
$ = x^3 +3.x^2.1 + 3.x.1^2 + 1^3$
$ = (x+1)³$