Áp dụng công thức $a^2-b^2=(a-b)(a+b)$
$17:1)3^2-x^2$
$=(3-x)(3+x)$
$2)x^2-36$
$=x^2-6^2$
$=(x-6)(x+6)$
$3)y^2-1$
$=(y-1)(y+1)$
$4)25-y^2$
$=5^2-y^2$
$=(5-y)(5+y)$
$18)4)(1+2x)(1-2x)$
$=1-4x^2$
$5)-(2x+3)(3-2x)$
$=-(3+2x)(3-2x)$
$=-(9-4x^2)$
$=4x^2-9$
$6)-(5x-3)(3+5x)$
$=-(5x-3)(5x+3)$
$=-(25x^2-9)$
$=9-25x^2$
$7)-\bigg(3x-\dfrac{2}{5}\bigg)\bigg(3x+\dfrac{2}{5}\bigg)$
$=-\bigg(9x^2-\dfrac{4}{25}\bigg)$
$=\dfrac{4}{25}-9x^2$
