Đáp án:
$b)5x(x-3)^2+5(x-4)(x+4)-5x^3=-5x(6x-1)$
$5x(x^2-6x+9)+5(x^2-16)-5x^3=-30x^2+5x$
$5x^3-30x^2+45x+5x^2-80-5x^3=-30x^2+5x$
$-25x^2+45x-80=-30x^2+5x$
$-25x^2+45x-80+30x^2-5x=0$
$5x^2+40x-80=0$
$5(x^2+8x-16)=0$
$x^2+8x-16=0$
$\text{ $x∈\{4-4\sqrt{2};-4+4\sqrt{2}\}$ }$
HỌC TỐT NHA~~