$1/(x-10)^2-x(x+80)$
$=x^2-20x+100-x^2-80x$
$=-100x+100$
$=-100(x-1)$
Với $x=0,98⇒-100.(0,98-1)=2$
$2/(2x+9)^2-x(4x+31)$
$=4x^2+36x+81-4x^2-31x$
$=5x+81$
Với $x=-16,2⇒5.(-16,2)+81=0$
$3/4x^2-28x+49$
$=(2x)^2-2.2.7x+7^2$
$=(2x-7)^2$
Với $x=4⇒(2.4-7)^2=1$
$4/x^3-9x^2+27x-27$
$=x^3-3.3x^2+3.3^2x-3^3$
$=(x-3)^3$
Với $x=5⇒(5-3)^3=8$
$5/9x^2+42x+49$
$=(3x)^2+2.3.7x+7^2$
$=(3x+7)^2$
Với $x=1⇒(3.1+7)^2=100$
$6/25x^2-2xy+\dfrac{1}{25}y^2$
$=(5x)^2-2.5.\dfrac{1}{5}xy+(\dfrac{1}{5}y)^2$
$=(5x-\dfrac{1}{5}y)^2$
Với $x=-\dfrac{1}{5},y=-5⇒(5.-\dfrac{1}{5}-\dfrac{1}{5}.-5)^2=0$
$7/27+(x-3)(x^2+3x+9)$
$=27+x^3-27$
$=x^3$
Với $x=-3⇒x=-27$
$8/x^3+3x^2+3x+1$
$=x^3+3.1.x^2+3.1^2.x+1^3$
$=(x+1)^3$
Với $x=99⇒(99+1)^3=1000000$
