Tìm x biết a) 5x ( x – 3 )^2 + 5 ( x – 4 ) ( x + 4 ) – 5x^3 = -5x ( 6x – 1 )

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{ $x∈\{4-4\sqrt{2};-4+4\sqrt{2}\}$ }$

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